click pay now in paypal page update my database too

I have a table to store my credit from user has been paid.

credit
  +id
  +amount

My goal is when you user click on button pay now in paypal page I have abiltity to update my table the amount field.

Anybody know the ways to do this,

thanks

EDIT: look at this code

http://www.micahcarrick.com/04-19-2005/php-paypal-ipn-integration-class.html

If the button goes off to PayPal, you'll need to intercept the request first, either with AJAX, or send to a PHP script, and then send on through to PayPal.

PayPal's Instant Payment Notification protocol is usually what's used to do this.