with PEAR in an O'Reilly article.
"You have probably seen the CAPTCHA project in action at some of your Web destinations. Its principal tool is a randomly created image that contains a phrase unmentioned in computer-readable text on the rendered page. The form asks the user to provide the phrase. If the form post does not contain the correct phrase, you can safely assume either the human made a user error, or it wasn't a human at all.
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