On PHPBuilder.com today there's a new tutorial posted helping you create a simple CAPTCHA image you can use in your forms to help to somewhat secure them from scripts out there that might abuse them.
CAPTCHA protection is used on web site forms and works by generating a random string, writing it to an image, then storing the string inside of a session. Then, the site checks the CAPTCHA when the form is submitted. In this article, you will learn how to make your own simple CAPTCHA image using PHP. You will create the CAPTCHA image step by step.
Their example uses numbers, lowercase letters and a few other characters to create the distorted image. The tutorial shows you how to take this character set, pull out a few randomized values and push them into a dynamically created image with PHP's built-in GD functionality. They also include a brief script showing how to use it in the form for validation of the submission.