IPN (Instant Payment Notification): notify_url\r\nInstant Payment Notification consists of three parts:\r\n1. A customer completed his order using paypal payment in your site.\r\n2. PayPal POSTs FORM variables to a URL you specify that runs a program to process the variables.\r\n3. You validate the notification.\r\n4. A customer payment or a refund triggers IPN. This payment can be via Website Payments Standard;FORMs or via the PayPal Web Services APIs for Express Checkout, MassPay, or RefundTransaction. If the payment has a\u201cPending\u201d status, you receive another IPN when the payment clears, fails,or is denied.\r\n5. PayPal posts HTML FORM variables to a program at a URL you specify. You can specify this URL either in your account profile or with the notify_url variable on each transaction. This post is the heart of IPN. Included in the notification is the customer\u2019s payment information (such as customer name,payment amount). When your server receives a notification, it must process the incoming data.\r\n6. Your server must then validate the notification to ensure that it is legitimate.