Hello, PHP World!

Example: Working with Variables

<?php
// String variables
$name = "John Doe";
$city = 'New York';
$greeting = "Hello, $name";  // Variable interpolation

// Integer variables
$age = 25;
$quantity = 100;
$temperature = -5;

// Float variables
$price = 19.99;
$pi = 3.14159;
$taxRate = 0.08;

// Boolean variables
$isLoggedIn = true;
$hasPermission = false;

// Array variables
$colors = array("red", "green", "blue");
$person = ["name" => "Alice", "age" => 30, "city" => "London"];

// NULL variable
$noValue = NULL;

// Output variables
echo "Name: $name<br>";
echo "Age: $age years<br>";
echo "Price: \$$price<br>";
echo "Logged in: " . ($isLoggedIn ? 'Yes' : 'No') . "<br>";

// Type checking
echo "Type of name: " . gettype($name) . "<br>";
echo "Type of age: " . gettype($age) . "<br>";
echo "Is array? " . (is_array($colors) ? 'Yes' : 'No') . "<br>";

// Constants (cannot be changed)
define("SITE_NAME", "MyWebsite");
const MAX_USERS = 100;
echo SITE_NAME . " can have " . MAX_USERS . " users.<br>";

// Variable variables
$category = "books";
$$category = "PHP Programming";  // Creates $books
echo "Category: $category, Value: $books<br>";
?>

Example: Operators & Control Flow

<?php
// Arithmetic operators
$a = 10;
$b = 3;
echo "Addition: " . ($a + $b) . "<br>";
echo "Subtraction: " . ($a - $b) . "<br>";
echo "Multiplication: " . ($a * $b) . "<br>";
echo "Division: " . ($a / $b) . "<br>";
echo "Modulus: " . ($a % $b) . "<br>";
echo "Exponent: " . ($a ** $b) . "<br>";

// Comparison operators
$x = 5;
$y = "5";
echo "x == y: " . ($x == $y) . "<br>";   // true (value comparison)
echo "x === y: " . ($x === $y) . "<br>"; // false (strict comparison)

// Logical operators
$hasAccount = true;
$isPremium = false;

if ($hasAccount && !$isPremium) {
    echo "Free account user<br>";
}

if ($hasAccount || $isPremium) {
    echo "Has access to basic features<br>";
}

// String concatenation
$firstName = "John";
$lastName = "Doe";
$fullName = $firstName . " " . $lastName;
echo "Full Name: $fullName<br>";

// if-elseif-else statement
$score = 85;

if ($score >= 90) {
    $grade = "A";
    echo "Excellent!<br>";
} elseif ($score >= 80) {
    $grade = "B";
    echo "Good job!<br>";
} elseif ($score >= 70) {
    $grade = "C";
    echo "Fair.<br>";
} else {
    $grade = "F";
    echo "Needs improvement.<br>";
}

echo "Grade: $grade<br>";

// Switch statement
$day = "Monday";

switch($day) {
    case "Monday":
        echo "Start of work week<br>";
        break;
    case "Friday":
        echo "Weekend is coming!<br>";
        break;
    case "Saturday":
    case "Sunday":
        echo "It's the weekend!<br>";
        break;
    default:
        echo "Regular day<br>";
}

// Ternary operator
$age = 20;
$status = ($age >= 18) ? "Adult" : "Minor";
echo "Status: $status<br>";

// Null coalescing operator
$username = $_GET['user'] ?? 'guest';
echo "Username: $username<br>";

// Spaceship operator (PHP 7+)
echo "1 <=> 1: " . (1 <=> 1) . "<br>"; // 0
echo "1 <=> 2: " . (1 <=> 2) . "<br>"; // -1
echo "2 <=> 1: " . (2 <=> 1) . "<br>"; // 1
?>

Example: Loops & Arrays

<?php
// For loop
echo "Counting 1 to 5: ";
for($i = 1; $i <= 5; $i++) {
    echo "$i ";
}
echo "<br>";

// While loop
echo "While loop countdown: ";
$count = 5;
while($count > 0) {
    echo "$count ";
    $count--;
}
echo "<br>";

// Do-while loop
echo "Do-while (executes at least once): ";
$num = 1;
do {
    echo "$num ";
    $num++;
} while($num <= 3);
echo "<br>";

// Indexed array
$colors = ["Red", "Green", "Blue", "Yellow"];
echo "Colors array: ";
foreach($colors as $color) {
    echo "$color ";
}
echo "<br>";

// Associative array
$student = [
    "name" => "Alice",
    "age" => 22,
    "grade" => "A",
    "city" => "New York"
];

echo "Student information:<br>";
foreach($student as $key => $value) {
    echo ucfirst($key) . ": $value<br>";
}

// Multidimensional array
$students = [
    ["name" => "John", "grade" => "A"],
    ["name" => "Sarah", "grade" => "B"],
    ["name" => "Mike", "grade" => "A"]
];

echo "<h4>Student List:</h4>";
foreach($students as $student) {
    echo "Name: {$student['name']}, Grade: {$student['grade']}<br>";
}

// Array functions
$numbers = [5, 2, 8, 1, 9];

// Sorting
sort($numbers);
echo "Sorted numbers: " . implode(", ", $numbers) . "<br>";

// Array filtering
$evenNumbers = array_filter($numbers, function($num) {
    return $num % 2 == 0;
});
echo "Even numbers: " . implode(", ", $evenNumbers) . "<br>";

// Array mapping
$squared = array_map(function($num) {
    return $num * $num;
}, $numbers);
echo "Squared numbers: " . implode(", ", $squared) . "<br>";

// Array reduce (sum)
$sum = array_reduce($numbers, function($carry, $item) {
    return $carry + $item;
}, 0);
echo "Sum of numbers: $sum<br>";

// Array searching
$search = array_search(8, $numbers);
echo "Number 8 found at index: $search<br>";

// Array merging
$array1 = [1, 2, 3];
$array2 = [4, 5, 6];
$merged = array_merge($array1, $array2);
echo "Merged array: " . implode(", ", $merged) . "<br>";

// Nested loops
echo "<h4>Multiplication Table (1-3):</h4>";
for($i = 1; $i <= 3; $i++) {
    for($j = 1; $j <= 3; $j++) {
        echo ($i * $j) . "\t";
    }
    echo "<br>";
}
?>

Example: Functions & Form Handling

<?php
// Basic function
function greet($name) {
    return "Hello, $name!";
}
echo greet("John") . "<br>";

// Function with default parameter
function calculateArea($length, $width = 10) {
    return $length * $width;
}
echo "Area (default width): " . calculateArea(5) . "<br>";
echo "Area (custom width): " . calculateArea(5, 8) . "<br>";

// Function with type declarations (PHP 7+)
function addNumbers(int $a, int $b): int {
    return $a + $b;
}
echo "Sum: " . addNumbers(5, 3) . "<br>";

// Function returning multiple values (using array)
function calculateStats($numbers) {
    return [
        'count' => count($numbers),
        'sum' => array_sum($numbers),
        'average' => array_sum($numbers) / count($numbers),
        'min' => min($numbers),
        'max' => max($numbers)
    ];
}

$nums = [10, 20, 30, 40, 50];
$stats = calculateStats($nums);
echo "Count: {$stats['count']}, Sum: {$stats['sum']}, Avg: {$stats['average']}<br>";

// Anonymous function (closure)
$multiply = function($x, $y) {
    return $x * $y;
};
echo "Product: " . $multiply(4, 5) . "<br>";

// Variable-length arguments (PHP 5.6+)
function sumAll(...$numbers) {
    return array_sum($numbers);
}
echo "Total: " . sumAll(1, 2, 3, 4, 5) . "<br>";

// ============================================
// FORM HANDLING EXAMPLE
// ============================================

// Check if form is submitted
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // Sanitize input
    $name = htmlspecialchars(trim($_POST['name'] ?? ''));
    $email = filter_var($_POST['email'] ?? '', FILTER_SANITIZE_EMAIL);
    $age = intval($_POST['age'] ?? 0);
    $message = htmlspecialchars(trim($_POST['message'] ?? ''));
    
    // Validation
    $errors = [];
    
    if (empty($name)) {
        $errors[] = "Name is required";
    } elseif (strlen($name) < 2) {
        $errors[] = "Name must be at least 2 characters";
    }
    
    if (empty($email)) {
        $errors[] = "Email is required";
    } elseif (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        $errors[] = "Invalid email format";
    }
    
    if ($age < 1 || $age > 120) {
        $errors[] = "Age must be between 1 and 120";
    }
    
    if (empty($message)) {
        $errors[] = "Message is required";
    } elseif (strlen($message) < 10) {
        $errors[] = "Message must be at least 10 characters";
    }
    
    // If no errors, process form
    if (empty($errors)) {
        $success = true;
        // In real application: save to database, send email, etc.
    }
}
?>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Contact Form</title>
    <style>
        .container { max-width: 600px; margin: 20px auto; padding: 20px; }
        .success { background: #d4edda; color: #155724; padding: 15px; border-radius: 5px; margin-bottom: 20px; }
        .error { background: #f8d7da; color: #721c24; padding: 15px; border-radius: 5px; margin-bottom: 20px; }
        .form-group { margin-bottom: 15px; }
        label { display: block; margin-bottom: 5px; font-weight: bold; }
        input, textarea, select {
            width: 100%; padding: 8px; border: 1px solid #ddd; border-radius: 4px;
        }
        button { background: #007bff; color: white; padding: 10px 20px; border: none; border-radius: 4px; cursor: pointer; }
        button:hover { background: #0056b3; }
    </style>
</head>
<body>
    <div class="container">
        <h2>Contact Form Example</h2>
        
        <?php if(isset($success) && $success): ?>
            <div class="success">
                <h3>✅ Form Submitted Successfully!</h3>
                <p>Thank you for your message, <strong><?php echo $name; ?></strong>!</p>
                <p>We'll get back to you at <strong><?php echo $email; ?></strong>.</p>
            </div>
        <?php endif; ?>
        
        <?php if(!empty($errors)): ?>
            <div class="error">
                <h3>❌ Please fix the following errors:</h3>
                <ul>
                    <?php foreach($errors as $error): ?>
                        <li><?php echo $error; ?></li>
                    <?php endforeach; ?>
                </ul>
            </div>
        <?php endif; ?>
        
        <form method="POST" action="">
            <div class="form-group">
                <label for="name">Full Name:*</label>
                <input type="text" id="name" name="name" required 
                       value="<?php echo htmlspecialchars($name ?? ''); ?>">
            </div>
            
            <div class="form-group">
                <label for="email">Email Address:*</label>
                <input type="email" id="email" name="email" required
                       value="<?php echo htmlspecialchars($email ?? ''); ?>">
            </div>
            
            <div class="form-group">
                <label for="age">Age:*</label>
                <input type="number" id="age" name="age" min="1" max="120" required
                       value="<?php echo $age ?? ''; ?>">
            </div>
            
            <div class="form-group">
                <label for="message">Message:*</label>
                <textarea id="message" name="message" rows="5" required>
                    <?php echo htmlspecialchars($message ?? ''); ?>
                </textarea>
            </div>
            
            <button type="submit">Send Message</button>
            <button type="reset" style="background: #6c757d;">Clear Form</button>
        </form>
        
        <?php if(isset($success) && $success): ?>
            <div style="margin-top: 20px; padding: 15px; background: #f8f9fa; border-radius: 5px;">
                <h4>Submitted Data:</h4>
                <pre><?php 
                    print_r([
                        'name' => $name,
                        'email' => $email,
                        'age' => $age,
                        'message' => $message
                    ]); 
                ?></pre>
            </div>
        <?php endif; ?>
    </div>
</body>
</html>

Example: Session & Cookie Management

<?php
// Start session at the beginning
session_start();

// Initialize session data
if (!isset($_SESSION['visit_count'])) {
    $_SESSION['visit_count'] = 1;
    $_SESSION['first_visit'] = date('Y-m-d H:i:s');
} else {
    $_SESSION['visit_count']++;
}

// Set last visit time
$_SESSION['last_visit'] = date('Y-m-d H:i:s');

// Cookie management
$cookie_name = "user_preference";
$cookie_value = "dark_mode";

// Set cookie (expires in 30 days)
if (!isset($_COOKIE[$cookie_name])) {
    setcookie($cookie_name, $cookie_value, time() + (86400 * 30), "/");
}

// Handle login
if ($_SERVER["REQUEST_METHOD"] == "POST" && isset($_POST['login'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];
    
    // Simple authentication (in real app, use database with password_hash())
    if ($username === "admin" && $password === "password123") {
        $_SESSION['loggedin'] = true;
        $_SESSION['username'] = $username;
        $_SESSION['login_time'] = time();
        
        // Regenerate session ID for security
        session_regenerate_id(true);
        
        // Set remember me cookie
        if (isset($_POST['remember'])) {
            setcookie("remember_user", $username, time() + (86400 * 30), "/");
        }
        
        header("Location: " . $_SERVER['PHP_SELF']);
        exit();
    } else {
        $login_error = "Invalid username or password";
    }
}

// Handle logout
if (isset($_GET['logout'])) {
    // Clear session data
    $_SESSION = array();
    
    // Destroy session cookie
    if (isset($_COOKIE[session_name()])) {
        setcookie(session_name(), '', time()-3600, '/');
    }
    
    // Destroy remember me cookie
    setcookie("remember_user", "", time()-3600, "/");
    
    // Destroy session
    session_destroy();
    
    header("Location: " . $_SERVER['PHP_SELF']);
    exit();
}

// Check if user is remembered
if (isset($_COOKIE['remember_user']) && !isset($_SESSION['loggedin'])) {
    $_SESSION['loggedin'] = true;
    $_SESSION['username'] = $_COOKIE['remember_user'];
}
?>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Session & Cookie Demo</title>
    <style>
        body { font-family: Arial, sans-serif; padding: 20px; max-width: 800px; margin: 0 auto; }
        .info-box { background: #e7f3ff; padding: 20px; border-radius: 8px; margin-bottom: 20px; border-left: 4px solid #3498db; }
        .login-box { background: #f8f9fa; padding: 20px; border-radius: 8px; border: 1px solid #ddd; }
        .form-group { margin-bottom: 15px; }
        label { display: block; margin-bottom: 5px; font-weight: bold; }
        input[type="text"], input[type="password"] { width: 100%; padding: 8px; border: 1px solid #ddd; border-radius: 4px; }
        button { background: #3498db; color: white; padding: 10px 20px; border: none; border-radius: 4px; cursor: pointer; }
        button:hover { background: #2980b9; }
        .error { color: #e74c3c; margin-bottom: 15px; }
        .success { color: #27ae60; margin-bottom: 15px; }
    </style>
</head>
<body>
    <h1>Session & Cookie Management Demo</h1>
    
    <div class="info-box">
        <h3>Session Information</h3>
        <?php if(isset($_SESSION['loggedin']) && $_SESSION['loggedin']): ?>
            <p class="success">✅ Welcome back, <strong><?php echo $_SESSION['username']; ?></strong>!</p>
            <p>Login time: <?php echo date('H:i:s', $_SESSION['login_time']); ?></p>
            <a href="?logout=true"><button>Logout</button></a>
        <?php else: ?>
            <p>You are not logged in</p>
        <?php endif; ?>
        
        <p>Visit count: <strong><?php echo $_SESSION['visit_count']; ?></strong></p>
        <p>First visit: <?php echo $_SESSION['first_visit']; ?></p>
        <p>Last visit: <?php echo $_SESSION['last_visit']; ?></p>
        <p>Session ID: <code><?php echo session_id(); ?></code></p>
    </div>
    
    <div class="info-box">
        <h3>Cookie Information</h3>
        <p>Theme preference: <strong>
            <?php echo isset($_COOKIE[$cookie_name]) ? $_COOKIE[$cookie_name] : 'Not set'; ?>
        </strong></p>
        <p>Remembered user: <strong>
            <?php echo isset($_COOKIE['remember_user']) ? $_COOKIE['remember_user'] : 'None'; ?>
        </strong></p>
        <p>All cookies:</p>
        <pre><?php print_r($_COOKIE); ?></pre>
    </div>
    
    <?php if(!isset($_SESSION['loggedin']) || !$_SESSION['loggedin']): ?>
    <div class="login-box">
        <h3>Login Form</h3>
        
        <?php if(isset($login_error)): ?>
            <p class="error">❌ <?php echo $login_error; ?></p>
        <?php endif; ?>
        
        <form method="POST" action="">
            <div class="form-group">
                <label for="username">Username:</label>
                <input type="text" id="username" name="username" required
                       value="<?php echo isset($_COOKIE['remember_user']) ? $_COOKIE['remember_user'] : ''; ?>">
            </div>
            
            <div class="form-group">
                <label for="password">Password:</label>
                <input type="password" id="password" name="password" required>
            </div>
            
            <div class="form-group">
                <label>
                    <input type="checkbox" name="remember">
                    Remember me for 30 days
                </label>
            </div>
            
            <button type="submit" name="login">Login</button>
        </form>
        
        <p style="margin-top: 15px; font-size: 0.9em; color: #666;">
            Use: <strong>admin</strong> / <strong>password123</strong> to login
        </p>
    </div>
    <?php endif; ?>
    
    <div style="margin-top: 30px; padding: 15px; background: #f8f9fa; border-radius: 5px;">
        <h4>All Session Data:</h4>
        <pre><?php print_r($_SESSION); ?></pre>
    </div>
</body>
</html>

Example: MySQL Database Operations

<?php
// Database configuration
$host = 'localhost';
$user = 'root';
$password = '';
$database = 'test_db';
$port = 3306;

// ============================================
// MySQLi Example
// ============================================
echo "<h3>MySQLi Example</h3>";

// Create connection
$conn = mysqli_connect($host, $user, $password, $database, $port);

// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}
echo "✅ Connected to MySQL database<br>";

// Create users table
$sql = "CREATE TABLE IF NOT EXISTS users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE,
    password VARCHAR(255) NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
)";

if (mysqli_query($conn, $sql)) {
    echo "✅ Users table created/verified<br>";
} else {
    echo "❌ Error creating table: " . mysqli_error($conn) . "<br>";
}

// Insert user with prepared statement
$username = "john_doe";
$email = "john@example.com";
$password_hash = password_hash("secure123", PASSWORD_DEFAULT);

$stmt = mysqli_prepare($conn, 
    "INSERT INTO users (username, email, password) VALUES (?, ?, ?)");
mysqli_stmt_bind_param($stmt, "sss", $username, $email, $password_hash);

if (mysqli_stmt_execute($stmt)) {
    echo "✅ User inserted successfully<br>";
    $last_id = mysqli_insert_id($conn);
    echo "📝 Last inserted ID: $last_id<br>";
} else {
    echo "❌ Error inserting user: " . mysqli_error($conn) . "<br>";
}

// Select users
$sql = "SELECT id, username, email, created_at FROM users";
$result = mysqli_query($conn, $sql);

if (mysqli_num_rows($result) > 0) {
    echo "<h4>Users in database:</h4>";
    echo "<table border='1' cellpadding='8' style='border-collapse: collapse;'>";
    echo "<tr><th>ID</th><th>Username</th><th>Email</th><th>Created At</th></tr>";
    
    while($row = mysqli_fetch_assoc($result)) {
        echo "<tr>";
        echo "<td>" . $row['id'] . "</td>";
        echo "<td>" . htmlspecialchars($row['username']) . "</td>";
        echo "<td>" . htmlspecialchars($row['email']) . "</td>";
        echo "<td>" . $row['created_at'] . "</td>";
        echo "</tr>";
    }
    echo "</table>";
} else {
    echo "No users found<br>";
}

// Close MySQLi connection
mysqli_close($conn);

// ============================================
// PDO Example
// ============================================
echo "<hr><h3>PDO Example</h3>";

try {
    // PDO Connection
    $dsn = "mysql:host=$host;dbname=$database;port=$port;charset=utf8mb4";
    $options = [
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
        PDO::ATTR_EMULATE_PREPARES => false,
    ];
    
    $pdo = new PDO($dsn, $user, $password, $options);
    echo "✅ PDO Connected successfully<br>";
    
    // Create products table
    $pdo->exec("CREATE TABLE IF NOT EXISTS products (
        id INT AUTO_INCREMENT PRIMARY KEY,
        name VARCHAR(100) NOT NULL,
        price DECIMAL(10, 2) NOT NULL,
        stock INT DEFAULT 0,
        created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
    )");
    
    echo "✅ Products table created/verified<br>";
    
    // Insert products using transaction
    $pdo->beginTransaction();
    try {
        $stmt = $pdo->prepare("INSERT INTO products (name, price, stock) VALUES (?, ?, ?)");
        
        $products = [
            ['Laptop', 999.99, 10],
            ['Mouse', 25.50, 50],
            ['Keyboard', 75.00, 30],
            ['Monitor', 299.99, 15]
        ];
        
        foreach ($products as $product) {
            $stmt->execute($product);
        }
        
        $pdo->commit();
        echo "✅ Products inserted successfully<br>";
        
    } catch (Exception $e) {
        $pdo->rollBack();
        echo "❌ Transaction failed: " . $e->getMessage() . "<br>";
    }
    
    // Select products with WHERE clause
    $stmt = $pdo->prepare("SELECT * FROM products WHERE price > ? ORDER BY price DESC");
    $min_price = 50;
    $stmt->execute([$min_price]);
    $products = $stmt->fetchAll();
    
    if (count($products) > 0) {
        echo "<h4>Products (price > $min_price):</h4>";
        echo "<table border='1' cellpadding='8' style='border-collapse: collapse;'>";
        echo "<tr><th>ID</th><th>Name</th><th>Price</th><th>Stock</th></tr>";
        
        foreach ($products as $product) {
            echo "<tr>";
            echo "<td>" . $product['id'] . "</td>";
            echo "<td>" . htmlspecialchars($product['name']) . "</td>";
            echo "<td>$" . number_format($product['price'], 2) . "</td>";
            echo "<td>" . $product['stock'] . "</td>";
            echo "</tr>";
        }
        echo "</table>";
    }
    
    // Search with LIKE
    $search_term = "%lap%";
    $stmt = $pdo->prepare("SELECT * FROM products WHERE name LIKE ?");
    $stmt->execute([$search_term]);
    $search_results = $stmt->fetchAll();
    
    if (count($search_results) > 0) {
        echo "<h4>Search results for '$search_term':</h4>";
        foreach ($search_results as $result) {
            echo htmlspecialchars($result['name']) . " - $" . $result['price'] . "<br>";
        }
    }
    
    // Aggregate functions
    $stmt = $pdo->query("SELECT 
        COUNT(*) as total_products,
        AVG(price) as avg_price,
        SUM(stock) as total_stock,
        MAX(price) as max_price,
        MIN(price) as min_price
        FROM products");
    
    $stats = $stmt->fetch();
    
    echo "<h4>Product Statistics:</h4>";
    echo "Total Products: " . $stats['total_products'] . "<br>";
    echo "Average Price: $" . number_format($stats['avg_price'], 2) . "<br>";
    echo "Total Stock: " . $stats['total_stock'] . "<br>";
    echo "Max Price: $" . number_format($stats['max_price'], 2) . "<br>";
    echo "Min Price: $" . number_format($stats['min_price'], 2) . "<br>";
    
    // Close PDO connection
    $pdo = null;
    
} catch(PDOException $e) {
    echo "❌ PDO Connection failed: " . $e->getMessage() . "<br>";
}
?>

Example: Basic OOP Implementation

<?php
// Basic Class Definition
class Person {
    // Properties (attributes)
    private $name;
    private $age;
    private $email;
    
    // Constructor - called when object is created
    public function __construct($name, $age, $email) {
        $this->name = $name;
        $this->age = $age;
        $this->email = $email;
        echo "Person object created: $name<br>";
    }
    
    // Destructor - called when object is destroyed
    public function __destruct() {
        echo "Person object destroyed: {$this->name}<br>";
    }
    
    // Getter methods
    public function getName() {
        return $this->name;
    }
    
    public function getAge() {
        return $this->age;
    }
    
    public function getEmail() {
        return $this->email;
    }
    
    // Setter methods
    public function setName($name) {
        $this->name = $name;
    }
    
    public function setAge($age) {
        if ($age > 0 && $age < 150) {
            $this->age = $age;
            return true;
        }
        return false;
    }
    
    public function setEmail($email) {
        if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
            $this->email = $email;
            return true;
        }
        return false;
    }
    
    // Display method
    public function displayInfo() {
        echo "<div style='border:1px solid #ddd; padding:10px; margin:10px;'>";
        echo "<h3>Person Information</h3>";
        echo "<p><strong>Name:</strong> {$this->name}</p>";
        echo "<p><strong>Age:</strong> {$this->age}</p>";
        echo "<p><strong>Email:</strong> {$this->email}</p>";
        echo "</div>";
    }
}

// Create Person objects
echo "<h3>Creating Person Objects</h3>";
$person1 = new Person("John Doe", 30, "john@example.com");
$person2 = new Person("Jane Smith", 25, "jane@example.com");

// Using getter methods
echo "<h4>Using Getter Methods</h4>";
echo "Person 1 Name: " . $person1->getName() . "<br>";
echo "Person 2 Age: " . $person2->getAge() . "<br>";

// Using setter methods
echo "<h4>Using Setter Methods</h4>";
$person1->setName("John Updated");
$person1->setAge(31);
echo "Updated Person 1: " . $person1->getName() . ", Age: " . $person1->getAge() . "<br>";

// Display information
$person1->displayInfo();
$person2->displayInfo();

// ============================================
// Inheritance Example
// ============================================

class Student extends Person {
    private $studentId;
    private $course;
    private $grades = [];
    
    public function __construct($name, $age, $email, $studentId, $course) {
        parent::__construct($name, $age, $email);
        $this->studentId = $studentId;
        $this->course = $course;
    }
    
    public function getStudentId() {
        return $this->studentId;
    }
    
    public function getCourse() {
        return $this->course;
    }
    
    public function addGrade($subject, $grade) {
        $this->grades[$subject] = $grade;
    }
    
    public function getGrades() {
        return $this->grades;
    }
    
    public function getAverageGrade() {
        if (empty($this->grades)) {
            return 0;
        }
        return array_sum($this->grades) / count($this->grades);
    }
    
    // Method overriding
    public function displayInfo() {
        parent::displayInfo();
        echo "<p><strong>Student ID:</strong> {$this->studentId}</p>";
        echo "<p><strong>Course:</strong> {$this->course}</p>";
        
        if (!empty($this->grades)) {
            echo "<p><strong>Grades:</strong></p>";
            echo "<ul>";
            foreach ($this->grades as $subject => $grade) {
                echo "<li>$subject: $grade</li>";
            }
            echo "</ul>";
            echo "<p><strong>Average:</strong> " . number_format($this->getAverageGrade(), 2) . "</p>";
        }
    }
}

echo "<hr><h3>Inheritance Example</h3>";
$student1 = new Student("Alice Johnson", 22, "alice@example.com", "S12345", "Computer Science");
$student1->addGrade("Mathematics", 85);
$student1->addGrade("Programming", 92);
$student1->addGrade("Database", 88);
$student1->displayInfo();

// ============================================
// Static Methods and Properties
// ============================================

class MathHelper {
    public static $pi = 3.14159;
    
    public static function add($a, $b) {
        return $a + $b;
    }
    
    public static function subtract($a, $b) {
        return $a - $b;
    }
    
    public static function multiply($a, $b) {
        return $a * $b;
    }
    
    public static function divide($a, $b) {
        if ($b == 0) {
            throw new Exception("Division by zero");
        }
        return $a / $b;
    }
    
    public static function circleArea($radius) {
        return self::$pi * $radius * $radius;
    }
}

echo "<hr><h3>Static Methods and Properties</h3>";
echo "5 + 3 = " . MathHelper::add(5, 3) . "<br>";
echo "10 - 4 = " . MathHelper::subtract(10, 4) . "<br>";
echo "Circle area (radius 5) = " . MathHelper::circleArea(5) . "<br>";

// ============================================
// Magic Methods
// ============================================

class MagicDemo {
    private $data = [];
    
    public function __set($name, $value) {
        $this->data[$name] = $value;
    }
    
    public function __get($name) {
        return $this->data[$name] ?? null;
    }
    
    public function __isset($name) {
        return isset($this->data[$name]);
    }
    
    public function __unset($name) {
        unset($this->data[$name]);
    }
    
    public function __toString() {
        return "MagicDemo object with data: " . print_r($this->data, true);
    }
}

echo "<hr><h3>Magic Methods</h3>";
$magic = new MagicDemo();
$magic->name = "Test";
$magic->value = 100;
echo $magic . "<br>";
echo "Name: " . $magic->name . "<br>";
?>

Example: Advanced OOP Features

<?php
// ============================================
// INTERFACES
// ============================================

interface Authenticable {
    public function login($username, $password);
    public function logout();
    public function isLoggedIn();
}

interface Loggable {
    public function log($message);
}

// ============================================
// ABSTRACT CLASS
// ============================================

abstract class DatabaseModel {
    protected $connection;
    
    abstract public function save();
    abstract public function delete();
    
    public function __construct() {
        // Simulate database connection
        $this->connection = true;
    }
    
    public function isConnected() {
        return $this->connection;
    }
}

// ============================================
// TRAITS
// ============================================

trait Timestampable {
    protected $createdAt;
    protected $updatedAt;
    
    public function setTimestamps() {
        $now = date('Y-m-d H:i:s');
        $this->createdAt = $now;
        $this->updatedAt = $now;
    }
    
    public function updateTimestamp() {
        $this->updatedAt = date('Y-m-d H:i:s');
    }
    
    public function getCreatedAt() {
        return $this->createdAt;
    }
    
    public function getUpdatedAt() {
        return $this->updatedAt;
    }
}

trait SoftDeletes {
    protected $deletedAt = null;
    
    public function delete() {
        $this->deletedAt = date('Y-m-d H:i:s');
        return "Soft deleted at: " . $this->deletedAt;
    }
    
    public function restore() {
        $this->deletedAt = null;
        return "Restored";
    }
    
    public function isDeleted() {
        return $this->deletedAt !== null;
    }
}

// ============================================
// CLASS USING INTERFACES, ABSTRACT CLASS, AND TRAITS
// ============================================

class User extends DatabaseModel implements Authenticable, Loggable {
    use Timestampable, SoftDeletes;
    
    private $username;
    private $passwordHash;
    private $isLoggedIn = false;
    
    public function __construct($username, $password) {
        parent::__construct();
        $this->username = $username;
        $this->passwordHash = password_hash($password, PASSWORD_DEFAULT);
        $this->setTimestamps();
    }
    
    // Interface methods
    public function login($username, $password) {
        if ($username === $this->username && password_verify($password, $this->passwordHash)) {
            $this->isLoggedIn = true;
            $this->log("User {$username} logged in");
            return true;
        }
        $this->log("Failed login attempt for {$username}");
        return false;
    }
    
    public function logout() {
        $this->isLoggedIn = false;
        $this->log("User {$this->username} logged out");
    }
    
    public function isLoggedIn() {
        return $this->isLoggedIn;
    }
    
    // Interface method
    public function log($message) {
        $timestamp = date('Y-m-d H:i:s');
        echo "[$timestamp] $message<br>";
    }
    
    // Abstract class methods
    public function save() {
        if ($this->isConnected()) {
            $this->updateTimestamp();
            $this->log("User '{$this->username}' saved to database");
            return "User '{$this->username}' saved successfully";
        }
        return "Error: Not connected to database";
    }
    
    // Override delete method from SoftDeletes trait
    public function delete() {
        $result = parent::delete(); // Call trait method
        $this->log("User '{$this->username}' soft deleted");
        return $result;
    }
    
    // Additional methods
    public function changePassword($oldPassword, $newPassword) {
        if (password_verify($oldPassword, $this->passwordHash)) {
            $this->passwordHash = password_hash($newPassword, PASSWORD_DEFAULT);
            $this->updateTimestamp();
            $this->log("Password changed for user '{$this->username}'");
            return true;
        }
        return false;
    }
    
    public function getUsername() {
        return $this->username;
    }
}

// ============================================
// NAMESPACES
// ============================================

namespace App\Models;

class Product {
    private $name;
    private $price;
    
    public function __construct($name, $price) {
        $this->name = $name;
        $this->price = $price;
    }
    
    public function getName() {
        return $this->name;
    }
    
    public function getPrice() {
        return $this->price;
    }
}

// ============================================
// TYPE DECLARATIONS (PHP 7+)
// ============================================

declare(strict_types=1);

class Calculator {
    public function add(int $a, int $b): int {
        return $a + $b;
    }
    
    public function divide(float $a, float $b): float {
        if ($b == 0) {
            throw new InvalidArgumentException("Division by zero");
        }
        return $a / $b;
    }
    
    public function concat(string $a, string $b): string {
        return $a . $b;
    }
}

// ============================================
// ANONYMOUS CLASSES (PHP 7+)
// ============================================

$logger = new class {
    public function log($message) {
        echo "[Anonymous Class] $message<br>";
    }
};

// ============================================
// DEMONSTRATION
// ============================================

echo "<h3>Advanced OOP Features Demonstration</h3>";

// Create and use User object
$user = new User("admin", "securepassword");
echo "<h4>User Operations</h4>";

// Login
if ($user->login("admin", "securepassword")) {
    echo "✅ Login successful<br>";
} else {
    echo "❌ Login failed<br>";
}

echo "Is logged in? " . ($user->isLoggedIn() ? 'Yes' : 'No') . "<br>";

// Save user
echo $user->save() . "<br>";

// Change password
if ($user->changePassword("securepassword", "newpassword123")) {
    echo "✅ Password changed successfully<br>";
} else {
    echo "❌ Password change failed<br>";
}

// Soft delete
echo $user->delete() . "<br>";
echo "Is deleted? " . ($user->isDeleted() ? 'Yes' : 'No') . "<br>";

// Restore
echo $user->restore() . "<br>";
echo "Is deleted after restore? " . ($user->isDeleted() ? 'Yes' : 'No') . "<br>";

// Logout
$user->logout();

// Calculator with type declarations
echo "<h4>Calculator with Type Declarations</h4>";
$calc = new Calculator();
echo "5 + 3 = " . $calc->add(5, 3) . "<br>";
echo "10.5 / 2.5 = " . $calc->divide(10.5, 2.5) . "<br>";
echo "Hello + World = " . $calc->concat("Hello", "World") . "<br>";

// Anonymous class
echo "<h4>Anonymous Class</h4>";
$logger->log("This is a test message");

// Namespace demonstration
echo "<h4>Namespace Demonstration</h4>";
$product = new App\Models\Product("Laptop", 999.99);
echo "Product: " . $product->getName() . ", Price: $" . $product->getPrice() . "<br>";

// ============================================
// POLYMORPHISM
// ============================================

interface Shape {
    public function area();
    public function perimeter();
}

class Circle implements Shape {
    private $radius;
    
    public function __construct($radius) {
        $this->radius = $radius;
    }
    
    public function area(): float {
        return pi() * $this->radius * $this->radius;
    }
    
    public function perimeter(): float {
        return 2 * pi() * $this->radius;
    }
}

class Rectangle implements Shape {
    private $width;
    private $height;
    
    public function __construct($width, $height) {
        $this->width = $width;
        $this->height = $height;
    }
    
    public function area(): float {
        return $this->width * $this->height;
    }
    
    public function perimeter(): float {
        return 2 * ($this->width + $this->height);
    }
}

echo "<h4>Polymorphism Example</h4>";
$shapes = [
    new Circle(5),
    new Rectangle(4, 6),
    new Circle(3)
];

foreach ($shapes as $shape) {
    echo "Area: " . number_format($shape->area(), 2) . 
         ", Perimeter: " . number_format($shape->perimeter(), 2) . "<br>";
}
?>

Example: Comprehensive Security Implementation

<?php
// ============================================
// SECURE SESSION CONFIGURATION
// ============================================

session_start([
    'cookie_httponly' => true,      // Prevent JavaScript access to cookies
    'cookie_secure' => true,        // Only send cookies over HTTPS
    'cookie_samesite' => 'Strict',  // Prevent CSRF attacks
    'use_strict_mode' => true,      // Enhanced session security
    'use_only_cookies' => true,     // Don't accept session IDs in URLs
    'cookie_lifetime' => 0,         // Session cookie expires when browser closes
]);

// Regenerate session ID periodically
if (!isset($_SESSION['last_regeneration'])) {
    session_regenerate_id(true);
    $_SESSION['last_regeneration'] = time();
} elseif (time() - $_SESSION['last_regeneration'] > 300) { // Every 5 minutes
    session_regenerate_id(true);
    $_SESSION['last_regeneration'] = time();
}

// ============================================
// INPUT VALIDATION AND SANITIZATION
// ============================================

class InputValidator {
    
    public static function sanitizeString($input) {
        $input = trim($input);
        $input = stripslashes($input);
        $input = htmlspecialchars($input, ENT_QUOTES | ENT_HTML5, 'UTF-8');
        return $input;
    }
    
    public static function validateEmail($email) {
        $email = filter_var($email, FILTER_SANITIZE_EMAIL);
        return filter_var($email, FILTER_VALIDATE_EMAIL) ? $email : false;
    }
    
    public static function validateURL($url) {
        // Remove dangerous protocols
        $dangerousProtocols = ['javascript:', 'data:', 'vbscript:'];
        foreach ($dangerousProtocols as $protocol) {
            if (stripos($url, $protocol) === 0) {
                return false;
            }
        }
        
        $url = filter_var($url, FILTER_SANITIZE_URL);
        if (!filter_var($url, FILTER_VALIDATE_URL)) {
            return false;
        }
        
        // Only allow http/https protocols
        $parsed = parse_url($url);
        if (!isset($parsed['scheme']) || !in_array(strtolower($parsed['scheme']), ['http', 'https'])) {
            return false;
        }
        
        return $url;
    }
    
    public static function validateInteger($number, $min = null, $max = null) {
        if (!is_numeric($number)) {
            return false;
        }
        
        $number = (int)$number;
        
        if ($min !== null && $number < $min) {
            return false;
        }
        
        if ($max !== null && $number > $max) {
            return false;
        }
        
        return $number;
    }
    
    public static function sanitizeHTML($html, $allowedTags = '<b><i><u><em><strong><code><pre>') {
        $html = trim($html);
        $html = strip_tags($html, $allowedTags);
        $html = htmlspecialchars($html, ENT_QUOTES | ENT_HTML5, 'UTF-8');
        return $html;
    }
}

// ============================================
// PASSWORD SECURITY
// ============================================

class PasswordSecurity {
    
    public static function hash($password) {
        return password_hash($password, PASSWORD_DEFAULT);
    }
    
    public static function verify($password, $hash) {
        return password_verify($password, $hash);
    }
    
    public static function needsRehash($hash) {
        return password_needs_rehash($hash, PASSWORD_DEFAULT);
    }
    
    public static function generateStrongPassword($length = 12) {
        $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!@#$%^&*()_-=+;:,.?';
        $password = '';
        $charsLength = strlen($chars) - 1;
        
        for ($i = 0; $i < $length; $i++) {
            $password .= $chars[random_int(0, $charsLength)];
        }
        
        return $password;
    }
    
    public static function checkStrength($password) {
        $score = 0;
        
        // Length check
        if (strlen($password) >= 8) $score++;
        if (strlen($password) >= 12) $score++;
        
        // Character variety
        if (preg_match('/[a-z]/', $password)) $score++;
        if (preg_match('/[A-Z]/', $password)) $score++;
        if (preg_match('/[0-9]/', $password)) $score++;
        if (preg_match('/[^A-Za-z0-9]/', $password)) $score++;
        
        return [
            'score' => $score,
            'strength' => $score >= 6 ? 'Strong' : ($score >= 4 ? 'Medium' : 'Weak')
        ];
    }
}

// ============================================
// CSRF PROTECTION
// ============================================

class CSRFProtection {
    private static $tokenName = 'csrf_token';
    
    public static function generateToken() {
        if (!isset($_SESSION[self::$tokenName])) {
            $_SESSION[self::$tokenName] = bin2hex(random_bytes(32));
        }
        return $_SESSION[self::$tokenName];
    }
    
    public static function validateToken($token) {
        if (!isset($_SESSION[self::$tokenName])) {
            throw new Exception('CSRF token not found in session');
        }
        
        // Use hash_equals for timing attack prevention
        if (!hash_equals($_SESSION[self::$tokenName], $token)) {
            throw new Exception('CSRF token validation failed');
        }
        
        return true;
    }
    
    public static function getTokenField() {
        $token = self::generateToken();
        return "<input type='hidden' name='" . self::$tokenName . "' value='$token'>";
    }
}

// ============================================
// SECURE DATABASE OPERATIONS
// ============================================

class SecureDatabase {
    private $pdo;
    
    public function __construct($host, $dbname, $username, $password) {
        try {
            $this->pdo = new PDO(
                "mysql:host=$host;dbname=$dbname;charset=utf8mb4",
                $username,
                $password,
                [
                    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
                    PDO::ATTR_EMULATE_PREPARES => false,
                    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
                    PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8mb4 COLLATE utf8mb4_unicode_ci"
                ]
            );
        } catch (PDOException $e) {
            // Log error but don't expose details to user
            error_log("Database connection failed: " . $e->getMessage());
            throw new Exception("Could not connect to database. Please try again later.");
        }
    }
    
    public function getUserByUsername($username) {
        $stmt = $this->pdo->prepare("SELECT * FROM users WHERE username = :username");
        $stmt->bindParam(':username', $username, PDO::PARAM_STR);
        $stmt->execute();
        return $stmt->fetch();
    }
    
    public function createUser($username, $email, $password) {
        $hash = PasswordSecurity::hash($password);
        
        $stmt = $this->pdo->prepare(
            "INSERT INTO users (username, email, password_hash, created_at) 
             VALUES (:username, :email, :hash, NOW())"
        );
        
        $stmt->bindParam(':username', $username, PDO::PARAM_STR);
        $stmt->bindParam(':email', $email, PDO::PARAM_STR);
        $stmt->bindParam(':hash', $hash, PDO::PARAM_STR);
        
        return $stmt->execute();
    }
    
    public function searchUsers($searchTerm, $limit = 10, $offset = 0) {
        $searchTerm = "%" . $searchTerm . "%";
        $stmt = $this->pdo->prepare(
            "SELECT id, username, email, created_at 
             FROM users 
             WHERE username LIKE :term OR email LIKE :term
             LIMIT :limit OFFSET :offset"
        );
        
        $stmt->bindParam(':term', $searchTerm, PDO::PARAM_STR);
        $stmt->bindParam(':limit', $limit, PDO::PARAM_INT);
        $stmt->bindParam(':offset', $offset, PDO::PARAM_INT);
        $stmt->execute();
        
        return $stmt->fetchAll();
    }
}

// ============================================
// SECURITY HEADERS
// ============================================

class SecurityHeaders {
    
    public static function setHeaders() {
        // Prevent clickjacking
        header('X-Frame-Options: DENY');
        
        // Enable XSS protection
        header('X-XSS-Protection: 1; mode=block');
        
        // Prevent MIME type sniffing
        header('X-Content-Type-Options: nosniff');
        
        // Referrer policy
        header('Referrer-Policy: strict-origin-when-cross-origin');
        
        // Content Security Policy (CSP)
        self::setCSP();
        
        // HSTS (should be configured in server, not PHP)
        // header('Strict-Transport-Security: max-age=31536000; includeSubDomains');
    }
    
    private static function setCSP() {
        $csp = [
            "default-src 'self'",
            "script-src 'self' 'unsafe-inline' 'unsafe-eval'",
            "style-src 'self' 'unsafe-inline'",
            "img-src 'self' data: https:",
            "font-src 'self'",
            "connect-src 'self'",
            "frame-ancestors 'none'",
            "form-action 'self'",
            "base-uri 'self'"
        ];
        
        header("Content-Security-Policy: " . implode('; ', $csp));
    }
}

// ============================================
// RATE LIMITING
// ============================================

class RateLimiter {
    private $maxAttempts;
    private $timeWindow;
    
    public function __construct($maxAttempts = 5, $timeWindow = 300) {
        $this->maxAttempts = $maxAttempts;
        $this->timeWindow = $timeWindow;
    }
    
    public function check($identifier) {
        $key = "rate_limit_$identifier";
        $now = time();
        
        if (!isset($_SESSION[$key])) {
            $_SESSION[$key] = [
                'attempts' => 1,
                'first_attempt' => $now,
                'last_attempt' => $now
            ];
            return true;
        }
        
        $data = $_SESSION[$key];
        
        // Reset if time window has passed
        if ($now - $data['first_attempt'] > $this->timeWindow) {
            $_SESSION[$key] = [
                'attempts' => 1,
                'first_attempt' => $now,
                'last_attempt' => $now
            ];
            return true;
        }
        
        // Check if exceeded max attempts
        if ($data['attempts'] >= $this->maxAttempts) {
            $waitTime = $this->timeWindow - ($now - $data['first_attempt']);
            return [
                'allowed' => false,
                'wait_time' => $waitTime,
                'message' => "Rate limit exceeded. Please wait $waitTime seconds."
            ];
        }
        
        // Increment attempts
        $data['attempts']++;
        $data['last_attempt'] = $now;
        $_SESSION[$key] = $data;
        
        return true;
    }
    
    public function getRemainingAttempts($identifier) {
        $key = "rate_limit_$identifier";
        if (!isset($_SESSION[$key])) {
            return $this->maxAttempts;
        }
        
        $data = $_SESSION[$key];
        $remaining = $this->maxAttempts - $data['attempts'];
        return max(0, $remaining);
    }
}

// ============================================
// DEMONSTRATION
// ============================================

echo "<h2>PHP Security Best Practices</h2>";

// Set security headers
SecurityHeaders::setHeaders();

// Generate CSRF token
$csrfToken = CSRFProtection::generateToken();
echo "<h3>CSRF Protection</h3>";
echo "CSRF Token Generated: " . substr($csrfToken, 0, 10) . "...<br>";
echo "CSRF Form Field: " . CSRFProtection::getTokenField() . "<br>";

// Password security demonstration
echo "<h3>Password Security</h3>";
$password = "MySecurePassword123!";
$hash = PasswordSecurity::hash($password);
echo "Password: $password<br>";
echo "Hash: " . substr($hash, 0, 20) . "...<br>";
echo "Verification: " . (PasswordSecurity::verify($password, $hash) ? '✅ Valid' : '❌ Invalid') . "<br>";

$strength = PasswordSecurity::checkStrength($password);
echo "Password Strength: {$strength['strength']} (Score: {$strength['score']}/6)<br>";

echo "Strong Password Example: " . PasswordSecurity::generateStrongPassword() . "<br>";

// Input validation demonstration
echo "<h3>Input Validation</h3>";
$testInput = "<script>alert('xss')</script>John O'Connor";
echo "Original: $testInput<br>";
echo "Sanitized: " . InputValidator::sanitizeString($testInput) . "<br>";

$email = "test@example.com<script>";
echo "Email Validation: " . (InputValidator::validateEmail($email) ? '✅ Valid' : '❌ Invalid') . "<br>";

// Rate limiting demonstration
echo "<h3>Rate Limiting</h3>";
$limiter = new RateLimiter(3, 60); // 3 attempts per minute
$ip = $_SERVER['REMOTE_ADDR'] ?? 'unknown';

$result = $limiter->check($ip);
if ($result === true) {
    echo "✅ Request allowed. Remaining attempts: " . $limiter->getRemainingAttempts($ip) . "<br>";
} else {
    echo "❌ " . $result['message'] . "<br>";
}

// Security checklist
echo "<h3>Security Checklist</h3>";
echo "<ul>";
echo "<li>✅ Input validation and sanitization</li>";
echo "<li>✅ Password hashing with password_hash()</li>";
echo "<li>✅ CSRF protection implemented</li>";
echo "<li>✅ SQL injection prevention with prepared statements</li>";
echo "<li>✅ Secure session configuration</li>";
echo "<li>✅ Security headers set</li>";
echo "<li>✅ Rate limiting implemented</li>";
echo "<li>✅ Error handling without information disclosure</li>";
echo "</ul>";

echo "<div class='key-concept' style='background: #d4edda; color: #155724; padding: 15px; margin: 20px 0;'>";
echo "<strong>Security Reminder:</strong> Always validate input, escape output, use HTTPS, keep software updated, and never trust user data. Security is an ongoing process, not a one-time setup.";
echo "</div>";
?>

Important: This security example demonstrates best practices. In production, always use HTTPS, keep PHP and all dependencies updated, conduct regular security audits, and monitor your application logs for suspicious activity.