So...I tried my hand at making my own captcha system for my under-construction website. The problem is that it is not really working. It blocks me out (if I don't type the text in the image), but it seems spambots aren't phased by it. You can find the page here: http://serfymcserf.com/guestbook.php Basically, I generate a random string of select characters. The string is then encrypted and sent to a script that decrypts it and displays it as an image. The encrypted string is also stored as a session variable, which, upon the submission of the guestbook, is cleared and a new one is generated. I think it's likely that a spambot is either reading the image, or there's a flaw in my method that is enabling the bot to read the unencrypted string somewhere. I'm pretty sure my code itself is pretty solid, but I guess I could be missing something. Any thoughts?