(PHP 5 >= 5.4.0)
SessionHandlerInterface::close — Close the session
Closes the current session. This function is automatically executed when closing the session, or explicitly via session_write_close().
此函数没有参数。
会话存储的返回值（通常成功返回 0，失败返回 1）。
svenr at selfhtml dot org (2013-04-30 21:29:50)
This method should release the exclusive lock that was acquired by the open() method.
Note that this will effectively serialize parallel requests, which affects performance for a single user session.
Not locking the resource would on the other hand allow for race conditions if two or more parallel requests change the session data.