i assume you are storing these tasks in a database? if so then i further assume that each task has some sort of date/time field to indicate when it was to be completed. write a script that querys the database for every task that is overdue and sends an email to the appropriate person. run that script via a cron every 24 hours. your crontab might look something like:
0 0 * * * /path/to/php /path/to/script.php >/path/to/output.log 2>/path/to/error.err
