I made a payment button that works with paypal. everything works, but I do not know how to show the outcome of the operation to the user. I am using this code in the "notify_url": https://github.com/paypal/ipn-code-samples/blob/master/paypal_ipn.php On this page I can handle errors and deciding on the message to be displayed on the page of "return" but as a step this error from the page of "notify_url" to that of "return"? Thanks.
ps: I tried ocn the $ _SESSION but it does not work.
You cannot directly print POST and GET in return variable, you will need to use PDT or IPN to get/retrieve the transaction and related information back.
If you want to get back the information instantly upon completion of payment, use PDT. If you want to get back the additional information upon payment completion, more recommended and reliable go for IPN..