Thanks for http://www.phpcaptcha.org/documentation/quickstart/ provide a cool php captcha code. One of my colleague supporting a php site which need CAPTCHA to prevent spamming but most of the example given needed page post back which is not so practical. I have implement an AJAX CAPTCHA for ASP in my previous post on AJAX ASP CAPTCHA.
Happy programming. Please let me know if you have any problem.