This is a tutorial on how to make your own (very simple) CAPTCHA protection using PHP to create a dynamically generated jpeg image. This is useful for preventing comment spam. I have a working demo available, and I also use this technique to protect my scratchpad and weblog comments. You can download a zip file with all the files used in this tutorial.
They introduce what a CAPTCHA image is and some of its common uses around the internet, including it's high popularity on blogging software. Their example script creates an image with a random five-character hex phrase, centered, and puts longer lines through it to make it harder to read. They provide not only the script to make the actual image, but also explain how to use it in an HTML form to make it userful.
If you like the tutorial, be sure to digg it to share it with others.