static function housekeep()
{
// delete sessions that have timed out and not logged out or
// deleted by check_session() and mark the user offline
$st = "SELECT session FROM active_sessions WHERE session_time<=" . (time() - CSession::$session_timeout) . "";
$streturn = mysql_query($st, CSession::$db_link) or die(mysql_error());
$stnum = mysql_numrows($streturn);
$i = 0;
while ($i < $stnum) {
CSession::check_session(mysql_result($streturn, 0, 0));
$i++;
}
}