if you are working with Paypal as your payment processor you certainly have heard of Instant Payment Notification supported by Paypal to instantly verify an customer purchase. By IPN turned on for your account, every time an customer purchases an item Paypal will post all the specific transaction details plus an security key to an url which is set from your side at your Paypal account profile. by receiving this transaction details, you will have to pass back the string to Paypal in order to prevent fraudsters from posting to your processing script. Paypal will then perform an check of your passed back data and if ok sends an verification of that transaction. exactly this routine handles the pp_ipn script at your side and also stores the relevant data foreach purchase in a single flat text file.