cre_captcha
Simple, single-file captcha library for both PHP and JSON / AJAX.
A basic, single-file captcha library designed for flexibility, ease of use, and customizability. This library has two separate interfaces -- the traditional PHP server-side interface, and a client-oriented AJAX / JSON interface which can be called at any time. This dual interface allows you to, for example, alert the user of an incorrect answer before your server processes that giant form they have spent the last half hour working on.
There are many different settings you can customize to...