How to preserve user information using PayPal ?

[Kane]

Let's hypothesize that I already have a site with users, whose names and passwords along with other data are already stored to a mySQL database.
One day I decide that I want to expand that site, but require payment for existing users in order to give them access to the new pages. I use PayPal in order to accept payments, in order to confirm that someone has already donated I use ipn.php which connects directly to PayPal and gets specific data. Thus I can not use cookies to know which user payed, and I am not able to update the row of the user, so they would not get what they payed for.
How can I pass data to PayPal in order to recognize who payed?

[freshnet]

The only way that's reliable is to update the row in the database or create a new table to reference the user with their payment. Why can't you do this?

[johnj]

...How can I pass data to PayPal in order to recognize who payed?

Read the paypal payment documentation, it has how to recognize the user.