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I would recommend against storing PHP code for your website in database fields. It will make it very difficult to maintain in the future.

Also, be sure that if someone has an active session from 11:50 PM until 12:05 AM the next day, that your $_SESSION['last-update'] values will work as expected. Timestamps are typically stored with date('Y-m-d H:i:s') or preferbly date('U') unixtimestamp for this reason.