PayPal has a huge API reference to integrate it’s services into any website. But from personal experience I found that the theory is far more simple than the practice. There are various methods to implement PayPal’s services including Soap webservices with ‘Express Checkout’ and ‘Website Payment Pro’. This tutorial will focus on ‘Website Payment Pro’ and the ‘IPN’ event listener.

IPN Services use an HTTP _POST or _GET conversation method. Your application sends a form request to PayPal’s web services which then send an HTTP request to an event handler. The event handler must then re-send the same post request back to authenticate the transaction as valid. Once returned and verified we can then process the transactions.

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