session_destroy() destroys all of the data associated with the current session. It does not unset any of the global variables associated with the session, or unset the session cookie. To use the session variables again, session_start() has to be called. 它會destroys all of the data associated with the current session，但是它既不注銷變量，又不注銷session cookie。那它做了什么？還有這個函數有什么用呢？