Uncaught exception thrown in shutdown function.

PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: DELETE FROM {semaphore} WHERE (value = :db_condition_placeholder_0) ; Array ( [:db_condition_placeholder_0] => 567215442687915266c6150.58385356 ) in lock_release_all() (line 269 of /home/qa/web/qa.sskru.ac.th/public_html/employ/includes/lock.inc).