Webmasters I need your help !
I want to build a registration system and I can't decide what will be the best way of doing it !

*I have a php form that the usr fill in .
*OnSubmit there is the DB record insertion.
*Moving to a page that the usr can choose the lengh of registration time and pay with paypal.
*Return to my site with a "thank you" page.
My Problem :
1. I want to send an email message with the details the user filled in to his address with the stored data in the DB. But after he paid.
2. I want my "thank you" page to present his details .
So .. Should I use a cookie ? to know who he is after he came bake from PP ?
And if so ..then what data to store in this cookie ?
Any help will be appriciate .. Thx.

You shouldn't need to use Cookies etc. Paypal operate IPN (Instant Payment Notification). This is a system that can interact with the scripts on your server and using this you can find out who paid and for what. Its very secure and reliable and by far the best way to integrate PayPal into your system.
http://www.paypal.com/cgi-bin/webscr...-intro-outside
There's example code and full specifications available there too.

Thanks man ..