checking the path from which someone has arrived at a specific script

Basically I want to check to see if a user has gone to PayPal.com and paid for a product before I disseminate the username and password through a script called thankyou.php. I tried using an if statement that checks the global variable $HTTP_REFERER: if($GLOBALS['HTTP_REFERER'] == "https://www.paypal.com/cgi-bin/webscr?__track=_xclick-flow:p/xcl/pay/buy-confirm:_xclick-payment-confirm-submit") { //access code that disseminates usernames and passwords } Unfortunately...This is not working. I went to the php manual and it basically stated that not all web sites will post the $HTTP_REFERER. I believe that is what is happening in this case. My question is: Is there another way to do this? I need to check whether or not they have been to paypal and bought a product. After there credit card has been approved they click on a link to go to thankyou.php for there username and password. Any Suggestions besides don't use PayPal?