php and crontab

hi,
could you tell me please how can i use crontab with php.
steps to explain for me how and where to put the crontab.
thanx

Could you elaborate a bit more?

i want to change my variable value in the midnight, i know that the solution is to use crontab with php. but i dont know how to use them or where to keep my cron code.
i want steps to use them.

In my case, I need to execute
turns.php
from within a crontab
turns.php in return accesses mysql

anyone?
ty4ur time