How can I validate a form using PHP? Example code

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $name = test_input($_POST["name"]);
    $email = test_input($_POST["email"]);
    $message = test_input($_POST["message"]);

    // Check if name is empty
    if (empty($name)) {
        $name_error = "Name is required";
    }

    // Check if email is valid
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        $email_error = "Invalid email format";
    }

    // Check if message is empty
    if (empty($message)) {
        $message_error = "Message is required";
    }

    // If no errors, send email
    if (empty($name_error) && empty($email_error) && empty($message_error)) {
        $to = "[email protected]";
        $subject = "Contact form submission";
        $headers = "From: $name <$email>";

        mail($to, $subject, $message, $headers);
        $success_message = "Message sent successfully";
    }
}

// Function to sanitize input
function test_input($data) {
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    return $data;
}