On the SitePoint PHP blog there's a new post by Mahul Jain looking at a simple way to create CAPTCHA images in PHP using the built-in GD libraries.

By now, we’ve all encountered captcha images in online forms. Captchas are a necessary evil, and this article will teach you how they’re made. Please note that while there are better, automatic third party solutions for captchas out there such as ReCaptcha, this tutorial aims merely to explain and demonstrate how such technology actually works. We won’t be explaining what captchas actually are, as it’s assumed to be common knowledge and already covered in greater detail elsewhere.

He walks you through the whole process of making the sample CAPTCHA (like this one):

• Display an empty image on the browser
• Create a shape
• Generate random lines
• Generate random dots
• Generate random text

There's screenshots all along the way too for reference as to what your result should look like. You can download the full code over on GitHub.