Simply PHP Anti-Bot Image

I need a simple script to prevent bots from autosubmitting form on my site. Basically, a php script which will generate random images with 8 letter string displayed in the image. User must input the 8 letter string and when the form is submitted, php should check that the 8 letter string corresponds to the string displayed in the image. Now, the trick is that once an 8 letter string is displayed, it should be put in a list of "used" strings and not be displayed again. This means that say first time user visits the string "1234" is displayed, user submits the form sucessfully. Now, if user tries to go back in the browser and submit the same form(with the same authentication string "1234") PHP should reject it. Don't really intend this to be hard. ## Deliverables 1) Complete and fully-functional working program(s) in executable form as well as complete source code of all work done. 2) Installation package that will install the software (in ready-to-run condition) on the platform(s) specified in this bid request. 3) Exclusive and complete copyrights to all work purchased. (No GPL, 3rd party components, etc. unless all copyright ramifications are explained AND AGREED TO by the buyer on the site). ## Platform php