In this article from PHPBuilder.com they take a look at how to integrate a "visual verification" system into your web forms to prevent spammers (CAPTCHA).

Many topics on the discussion forums deal with the verification of form data. Often it is checked to determine whether or not the submission is from a user or from a "bot", if the email address entered is a valid address, or if all the information that is required has been entered into the form.

While it's fairly easy to check to see if a form field is empty, determining if the posted information came from a real human is another task altogether. Most forms now include image verification for just this reason. This article will demonstrate how to create a simplified image verification system.

They walk you through the code, explainign each step of the way. They start with the creation of a random string, background, and font color for the CAPTCHA image to use. It's flexible enough to make either a string or just a word, too. Once the string is made, they set up the image to be written to and push each letter into it, rotating it to make it that much more difficult for scripts to try to understand its contents.