PHP中的自执行代码

I need a script which can exceute automatically when Current date reached. Forexample: when CurrentDate > DueDate Add Surcharge with Total Dues.

Note:

Add Surcharge with Only those customers who did'nt pay their monthly bill.

Can anyone give me some hint or any kind of Source. Thanks

write a cron every minute

* * * * * /path/to/php /var/www/html/a.php

or every 24th hour

0 0 * * * /path/to/php /var/www/html/a.php