Connecting a form with PayPal pay-button to MySQL database?

I am wondering how to connect my form with a PayPal's pay button to a MySQL database?

I have a form with username and e-mail address which my customers must fill in and then click a PayPal's pay button. After they fill in the information and click the pay-button, they will be directed to PayPal's payment site. What I want to happen is that when the payment is completed, the information they filled in gets stored on my database.

I hope it was understandable. Does anyone know how I should proceed? Any help is highly appreciated. Thanks!