question about cron jobs

i have apache/unix running and i have shell access to the web server. i was wondering if there was some way to add a cron job from php? like for example, i have an area of my site that allows users to set options...like how often to backup the site automatically. when they change this, obviously, the cron job has to be modified. i'm a little stumped as to how to do it. thanks, -jake