$_SESSION 无效请调用 Session_start
不调用的话时好时坏. 而且是要在每个使用的页面中都调用.要不就要在 php.ini 中设置 session.auto_start is set to 1
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注: If you are using cookie-based sessions, you must call session_start() before anything is outputted to the browser.
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Session support in PHP consists of a way to preserve certain data across subsequent accesses. This enables you to build more customized applications and increase the appeal of your web site.
A visitor accessing your web site is assigned an unique id, the so-called session id. This is either stored in a cookie on the user side or is propagated in the URL.
The session support allows you to register arbitrary numbers of variables to be preserved across requests. When a visitor accesses your site, PHP will check automatically (if session.auto_start is set to 1) or on your request (explicitly through session_start() or implicitly through session_register()) whether a specific session id has been sent with the request. If this is the case, the prior saved environment is recreated.
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实际上还有其他的一些限制,具体看 php 的文档吧.
session.auto_start is set to 1
