How can developers ensure that their PHP code effectively handles form validation, such as captcha verification?

Developers can ensure that their PHP code effectively handles form validation, such as captcha verification, by implementing server-side validation checks to verify the user input. This can include validating the captcha response against the expected value and displaying error messages if the validation fails.

// Validate captcha verification
$captcha_response = $_POST['captcha_response'];

// Expected captcha value
$expected_captcha = $_SESSION['captcha'];

if($captcha_response != $expected_captcha) {
    $errors[] = "Captcha verification failed. Please try again.";
}

// Display error messages
if(!empty($errors)) {
    foreach($errors as $error) {
        echo $error;
    }
}