How to properly link PayPal subscription to my database?

Question

I'm having some problems understanding the proper way to go about linking an account to someone's user account for my website, for my website I'm using a subscription based system that uses recurring payments. What I'm having a problem understanding is how to properly figure out which account that payment is relative to.

At first I though about using the payer_email to link an account and a paypal account together, however after somet thinking I realized that if the user was to change their e-mail then we'd run into some payment problems. (They would still pay, but their account would never be updated).

Then I thought, hmm, what about payer_id however some research brought up this statement:

As others have said, payer_id can be used to identify a Paypal account. HOWEVER! -- a single Paypal account can have several payer_ids associated with it, one for each credit card or funding source used by that account. Because of this, a given Paypal account does NOT map one-to-one to a single payer_id.

Then I thought about a transaction id; However this changes with each payment, so now I'm sitting here completely clueless as to the best way to map a paypal subscription to my account database.

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What I need to do:

1. When a payment comes in, find the account relative to the payment:
2. Update that accounts information.

Answer 1

Check the IPN of PAYPAL (Instant Payment Notification)

you can set the notification URL to return back some data.

FORM Eg:

<input type="hidden" name="cmd" value="_xclick-subscriptions">
<input type="hidden" name="return" value="test.com/paypal.php?action=upgrade_plan_success">
<input type="hidden" name="cancel_return" value="test.com/paypal.php?action=cancel">
<input type="hidden" name="notify_url" value="test.com/notify_paypal.php">
<input type="hidden" name="custom" value="$user_id"> //any unique thing about customer

when ever the payment is done it will notify you in url notify_paypal.php mention above. It will return some array, in that we have our custom value which we can use to manipulate database..

so imp is

<input type="hidden" name="custom" value="$user_id"> //any unique thing about customer

hope this will help you