New on NetTuts.com there's a tutorial about using cron with PHP and how to manage the jobs your server currently runs.

The cronTab, or “Cron Table”, is a Linux system process / daemon which facilitates the scheduling of repetitive tasks thereby easing up our day to day routine. In this tutorial, we’ll create a dynamic PHP class that, using a secure connection, provides us with a means to manipulate the cronTab!

They explain the cron format (time and script to run) and how to set up a crontab management class that uses the ssh extension for PHP to communicate with the remote server. Their class includes methods for setting up jobs, writing them out to the cron file, removing existing jobs and dropping the entire cron file at once.