if you are on Linux, you can use crontab -e command to edit crontab file. "man crontab" should show you a manual for crontab. But you are not able to run *.php file from crontab I think. You have to perform an http request to your http server (Apache) to perform an php script.
I do it through perl script, which is doing the same thing as HTML browsers do. One parametr is a web page for example "http://www.myserver.cz/script.php and this script performs the request. The perl script is called from crontab.
Zdenek