How can HTML5 form validation be used in conjunction with server-side PHP validation techniques to enhance the security of user input handling in web applications?

HTML5 form validation can be used to provide immediate feedback to users on their input, reducing the likelihood of submitting incorrect data. However, relying solely on client-side validation can leave the application vulnerable to malicious users bypassing the validation. To enhance security, server-side PHP validation should always be used to validate user input before processing it. This ensures that even if client-side validation is bypassed, the server will still validate the input before taking any action.

// Server-side PHP validation example
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $username = $_POST["username"];
    $password = $_POST["password"];
    
    // Validate input
    if (empty($username) || empty($password)) {
        echo "Username and password are required.";
    } else {
        // Process the data
        // Additional validation and sanitization can be performed here
    }
}