php - PayPal send and retrieve custom GET parameter -

my customers can pay via paypal link this: https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=gksjena5fsksw

is there way append parameter , later retrieve it?

actually know that user payed don't know who payed.

i'm storing user data before redirect paypal , give id. there way send id paypal , retrieve maybe parameter @ page paypal redirects when payment completed?