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[php 随机生成验证码]

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Release： 2016-06-23 13:24:26
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@header("Last-Modified: ".gmdate("D, d M Y H:i:s")." GMT");//标记内容最后修改时间@header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");//内容过期时间 @header("Cache-Control: no-cache, must-revalidate" );//强制不缓存@header("Pragma: no-cache" );//禁止本页被缓存
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一般情况下browser会保存曾经访问过的页面，二次访问时就直接提取缓存，不需要再向服务器请求了。

而这里需要保证每次的验证码不同，所以要强制浏览器每次都向服务器请求。

验证码（CAPTCHA）是“Completely Automated Public Turing test to tell Computers and Humans Apart”（全自动区分计算机和人类的图灵测试）的缩写，是一种区分用户是计算机还是人的公共全自动程序。

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[php 随机生成验证码]
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