What are best practices for handling external input in PHP forms to prevent security vulnerabilities?

To prevent security vulnerabilities in PHP forms due to external input, it is crucial to sanitize and validate user input before processing it. This can help prevent SQL injection, cross-site scripting (XSS), and other malicious attacks. One way to achieve this is by using PHP functions like htmlspecialchars() to escape special characters and filter_var() to validate input data.

// Sanitize and validate external input in PHP form
$name = isset($_POST['name']) ? htmlspecialchars($_POST['name']) : '';
$email = isset($_POST['email']) ? filter_var($_POST['email'], FILTER_VALIDATE_EMAIL) : '';
$message = isset($_POST['message']) ? htmlspecialchars($_POST['message']) : '';