You can build it yourself:- generate random text and put it in a session- put the text in an image (http://www.php.net/manual/en/function.imagecreate.php)- after submit, check if users' code matches to the one in the sessionStart on these 3 major steps and ask for details when needed.After this, you'll make more advanced captcha codes, with colors and various backgrounds to make it harder for the bots to read it.