On DevShed's PHP section today there's a new tutorial focusing on the creation of a CAPTCHA image to help prevent spam on the forms (and other input methods) of your website.
Spam is one of the biggest problems on the Internet. It is getting harder to fight with the advent of spam bots that visit websites and automatically fetch email addresses, fill out forms and do other nasty things, such as blog spam comments, that could degrade your integrity. Fortunately, using captcha can help. This article will show you how to implement captcha on your site.
CAPTCHA images are those small images, usually at the bottom of forms, that have distorted letters and numbers in an effort to prevent anything not human from submitting the form correctly. They're usually accompanied by a form field asking for the letters/numbers displayed in the image. In the tutorial they use PHP's built in GD support to create a script that builds the image from an inputted string.
They also give an option for a GD-less version, but it's not nearly as dynamic and wouldn't help as much as the image generated on the fly.