One of the best PHP Captchas that I have found, from their own site:

“This script generates images (known as “Captcha’s”) which contain security codes used for protecting a form from spam bots. By encoding a ‘password’ inside an image and asking the user to re-enter what they see you can verify the user is a human and not automated software submitting your form.”

PHP Captcha Page

It is easy to use, clearly coded, well commented and the how-to is amazing. In fact it is so awesome that I have used it in many scripts of my own, and I have even extended it to take colors through the query string. I set up a script here that uses Javascript to let you see what the captcha will look like when you pass it hex colors. I also have my modified code available in a zip file on the page.

My PHP Captcha, loaded to your colors as you type them

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