如何生成验证码的吗？

CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a simple test to determine if a user is a computer or a human. It’s is a type of challenge-response test used in computing to ensure that the response is not generated by a computer. CAPTCHA helps to prevent spam abuse on the websites. So if you use CAPTCHA on your web site forms, this can help in stopping some bots and making life harder for other bots in accessing or using your forms. In brief the CAPTCHA protection works by generating a random string, writing it to an image, then storing the string inside of a session or by some other method. This is then checked when the form is submitted.

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