To use paypal for your payment processor, you need to validate it because people can tamper the data they send to Paypal and the data that you receive from paypal. You can lost a lot of money, because user might not pay you for your product/ service. That’s why you need to validate this transaction by using Instant Payment Notification (IPN).

Instant Payment Notification (IPN) is a message service that notifies you of events related to PayPal transactions. You can use it to automate back-office and administrative functions, such as fulfilling orders, tracking customers, and providing status and other information related to a transaction.

IPN notifies you when an event occurs that affects a transaction. Typically, these events represent various kinds of payments; however, the events may also represent authorizations, Fraud Management Filter actions and other actions, such as refunds, disputes, and chargebacks.

You can download Paypal IPN sample code here.