Graphically displayed number to confirm user is a human

PHP General List, This is something I tried to find on Google, but I'm hindered by not knowing the proper terminology. On many sites that have contact or registration forms, they have a small image file with a randomly generated number shown in a distorted graphical fashion that is hard for a computer to scan and read. The purpose is that the site visitor has to type in the numbers into a field, in order to verify that it is, in fact, a human filling out the form. First, is there a term for these kinds of images, or that kind of verification system? What would be the best search terms to look for source scripts? Second, can anyone recommend a resource for that kind of PHP script. If I really worked at it, I could probably write the script to generate a small PNG image with a random number in it. But I haven't the faintest clue how to distort it so it's only legible to humans, nor how to correlate the image with a number that a user inputs into a form field. Any advice would be much appreciated. -- Dave M G