A McGraw Hill Book Plug-in PHP: 100 Power Solutions
By Robin Nixon (McGraw Hill 2010, ISBN 978-0071666596)

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Chapter 12: Plug-in 92 - Bypass Captcha

The reason you might use a Captcha system, such as Plug-in 33, is to prevent your website being overwhelmed with posts made by automated 'bots'. The trouble is, many people find it annoying to fill in a Captcha, so this plug-in is there to help. Using it your program can make a quick guess at whether the current user is human or not and, if it thinks so, bypass using a Captcha.

The Figure shows a web page that has been typed in directly and which therefore has no referring page, so it has returned a value of FALSE.