Some browsers cache the verification code, recruit the code expires when you reopen the page

Some browsers cache the CAPTCHA, causing the verification code to expire when the page is reopened.
Refreshing the browser is normal.
How to solve, what code can force the verification code re-loaded.
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Add a js random parameter after the generated address, url?r=math.random ()
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">
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You add a non-cached header to the CAPTCHA picture generator

Header ("Expires:mon, Jul 1997 05:00:00 GMT");    Date in the past
Header ("last-modified:".) Gmdate ("D, D M Y h:i:s"). "GMT");
Header ("Cache-control:no-cache, must-revalidate");  http/1.1
Header ("Pragma:no-cache");                          http/1.0

You can solve this problem once and for all without having to change the URL every time you use it.

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