To use reCAPTCHA with PHP, you can download reCAPTCHA PHP library. You will only need one file from there (recaptchalib.php). The other files are examples, readme and legal stuff -- they don't affect functionality.
เว็บไซท์สำหรับ PHP https://developers.google.com/recaptcha/docs/php
ที่มา : http://www.google.com/recaptcha
