<?php
require('recaptchalib.php');

define('RECAPTCHA_PUBLIC_KEY',  'public_key_xxx');
define('RECAPTCHA_PRIVATE_KEY', 'private_key_xxx');

function is_vertify_reptcha()
{
    $resp = recaptcha_check_answer (RECAPTCHA_PRIVATE_KEY,
            $_SERVER['REMOTE_ADDR'],
            $_POST['recaptcha_challenge_field'],
            $_POST['recaptcha_response_field']);

    return $resp->is_valid;
}

if ($_POST['post'] && is_vertify_reptcha()) {
    // save db or anything.
}
?>
<form method="post">
<p>... any input ...</p>
<?php echo recaptcha_get_html(RECAPTCHA_PUBLIC_KEY); // 會於此處出現 reCAPTCHA 驗證碼 ?>
<input type="submit" name="post">
</form>