When it comes to limiting the spam that comes through on your site (usually on the open comments), a CAPTCHA can do some of the best work. It provides a good alternative to convoluted techniques for restricting the commenting system of your site, and does it in a simple, easy for a human to use, kind of system. Implementing them, however, can be a bit tricky, so this new article on Dublish.com is here to help.
Captcha is the name for the images with random letters and some distortion that appears on each and every forum these days and even here on dublish. The purpose is to prevent the sites from spam.
Now I'm going to show you how to do this random picture generator in PHP. I assume that you have some basic PHP skillz before doing this. Before you start you need the GD library that allows you to create graphics in php.
The article steps you through the creation of the single-script functionality to generate the image. It's an image with a series of random characters on a gradient background created with the GD libraries. The code used in the CAPTCHA is stored in a session variable for validation later. He adds a few graphical touches to make it look a bit nicer, and finishes off with an example of its use.