What are common pitfalls when implementing a Captcha in a PHP-based forum?

Common pitfalls when implementing a Captcha in a PHP-based forum include not properly validating the Captcha input, using weak or easily bypassed Captcha mechanisms, and not securely storing or handling Captcha responses. To address these issues, ensure that the Captcha input is validated correctly, use a reliable and secure Captcha solution, and securely store Captcha responses to prevent misuse.

// Validate Captcha input
$captcha_input = $_POST['captcha_input'];
$captcha_session = $_SESSION['captcha_code'];

if(empty($captcha_input) || $captcha_input !== $captcha_session) {
    // Captcha input is incorrect, handle error
    echo "Captcha input is incorrect";
    exit;
}

// Securely store Captcha response
unset($_SESSION['captcha_code']);