A CAPTCHA is a challenge-response test that helps determine if the user is human. They are useful on signup forms, comment pages, and e-mail functions to prevent automated spam. There are some nice existing PHP functions for it, but most are a pain to setup or lack features such s audio support for the blind. reCaptcha is easy to use and is actually doing something useful with the user entry. reCaptcha helps in the process of digitizing books. It displays two words one known and one unknown that a user enters. If the user gets the first word right, it is assumed that they were also correct about the other.
reCaptcha is free and has several examples of how to use it with various programming languages. http://recaptcha.net/
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