我们都知道php有自己的一个默认session机制，就是你不设置任何session.handler的时候，它会用在临时目录下面存储文件名类似sess_5pjm50b8d40p4175iousv4hqd3的一堆session文件，但是session是有过期时间的，而这些文件显然不能无限增多，那么当session过期以后这些文件会被回收吗？是通过什么机制回收的呢？显然php本身干不了这个事情，因为它的生命周期在脚本执行完就结束了，没有长驻内存的程序来回收这些文件，难道就放任不管吗？
解决方案
在php.ini中可以看到以下配置行
; Define the probability that the 'garbage collection' process is started; on every session initialization.; The probability is calculated by using gc_probability/gc_divisor,; e.g. 1/100 means there is a 1% chance that the GC process starts; on each request.session.gc_probability = 1session.gc_divisor = 100; After this number of seconds, stored data will be seen as 'garbage' and; cleaned up by the garbage collection process.session.gc_maxlifetime = 1440
从注释可以看出是session垃圾回收的配置。 session.gc_probability/session.gc_divisor 表述有1%的概率回收大于session.gc_maxlifetime时间的过期文件。
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我们看 http://cn.php.net/manual/zh/sessionha... php文档对 session.gc 的说明
Cleans up expired sessions. Called randomly by PHP internally when a sessio starts or when session_start() is invoked. The frequency this is called is based on the session.gc_divisor and session.gc_probability configuration directives.This method wraps the internal PHP save handler defined in the session.save_handler ini setting that was set before this handler was set by session_set_save_handler().If this class is extended by inheritiance, calling the parent gc method will invoke the wrapper for this method and therefor invoke the associated internal callback. This allows this method to be overidden and or intercepted and filtered.For more information on what this method is expected to do, please refer to the documentation at SessionHandlerInterface::gc().
即：比如session_start()一次会话请求，会有1%的概率调用session的垃圾回收来以清理过期会话。