An easy gotcha here - the $_SESSIONS superglobal does not vanish because you call session_write_close. If subsequent to the write_close call if you manipulate the superglobal the changes will not be saved when the script exists. Likewise a call to session_regenrate_id will fail.
Closing the session and then manipulating session variables is not something many would do by intent. However, if your sessions suddenly start misbehaving, failing to record changes etc it is well worth checking that the cause is not this one!
