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Pro PHP Security: From Application Security Principles to the Implementation of XSS Defenses, Second Edition by Thomas Myer, Michael Southwell, Chris Snyder

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Creating an Effective Captcha Test Using PHP

Given the complexity and difficulty of using advanced captcha techniques, we confine ourselves to using the simplest kind of visual captcha, which presents a nondistorted (or minimally distorted) image of a word to a user, who must enter that word into a form. Such a challenge is capable of being used effectively in the vast majority of situations, and is unlikely to exclude many legitimate users.

Let an External Web Service Manage the Captcha for You

There can be no question that the simplest way to use a captcha is to let someone else do all the work. We are now seeing commercial web services that, for a fee, will allow you to incorporate a captcha challenge based on their servers into your website. ...

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