How do I automatically alert my php / mysql web application that a PayPal transaction has gone through?

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I have a client who will be selling access to an online service on their website. They would like to integrate a PayPal Buy Now button into the site... no problems there. However, they want their customers to have instant access to the online service that they are selling as soon as the payment is processed. That seems reasonable to me... the problem is that I can't seem to find a way to automate that. So my question is really this:

How can I automatically alert my php/mysql web app that a PayPal transaction has gone through?

All I would really need is like a callback process that triggers a php script to updates the customer's record in the database... right? Seems simple to me. But I can't seem to find a way to do it! Anyway, if you've accomplished anything similar with PayPal, I'd be grateful for your advice!


See Instant Payment Notification.