Just when you thought you were safe with the little CAPTCHA graphic on your site, something like this comes along - a method for decoding CAPTCHA images using only PHP.
This example shows a simple method of decoding "CAPTCHA" (Completely Automated Public Turing test to tell Computers and Humans Apart) correctly into usable variables for processing. There is no 100% guarantee that it would successfully decode the CAPTCHA (Maybe about 90% or more) but it is a start.
Their example pulls an image with overlapping characters to be parsed by their PHP script (needing the GD2 extension). They start off by taking the image into a graphic editor and breaking up the characters to create a "pixel library" of the letters (A-Z) and numbers (0-9) that service uses. This is then used by the PHP script to locate what letters/numbers might be present in the image.
They include in upload form to allow users to give it a try as well as a sample pixel library for the full series of letters/numbers and their points.