If You have a sudo access to your linux server :-
Then do the following
sudo crontab -e
This will open the cron tab for you on your server.
Next thing is you have to do a cron entry for the file which you want to execute
00 00 * * * /usr/local/bin/php "path of the php file which you want to execute"
00 00 * * * this will run your cron at midnight daily, means at 0hrs and 0mins