It depends on whether or not your web host offers a task scheduler (called CRON on Linux/Unix machines). Most PHP-based content management systems (Drupal, Moodle, PHPBB2, etc) that have some sort of periodic maintentance like sending out mailing lists, have a special script, typically called "cron.php", which must then be called from the cron application.

Failing that, the only other way to do it would be to set up a scheduled task on YOUR machine to call up the script in your web browser... which is pretty awkward.

For your calendar example, you would set up a CRON task to call your script once a day. The script would then check the database and see if there are any upcoming notifications that need to be sent out.

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