What are the best practices for securely handling user input in PHP, especially when it comes to form data?

When handling user input in PHP, especially form data, it is crucial to sanitize and validate the input to prevent security vulnerabilities such as SQL injection or cross-site scripting attacks. Use PHP functions like `htmlspecialchars()` to escape special characters and `filter_input()` to validate input. Additionally, always use prepared statements when interacting with a database to prevent SQL injection attacks.

// Sanitize and validate user input from a form
$user_input = $_POST['user_input'];

// Sanitize input to prevent XSS attacks
$sanitized_input = htmlspecialchars($user_input);

// Validate input using filter_input
$validated_input = filter_input(INPUT_POST, 'user_input', FILTER_SANITIZE_STRING);

// Use prepared statements when interacting with a database
$stmt = $pdo->prepare('SELECT * FROM users WHERE username = :username');
$stmt->bindParam(':username', $validated_input);
$stmt->execute();