paypal_ipn: Paypal payment notification processing

This class is usefull to process Instant Payment Notification (IPN) from Paypal.
See www.paypal.com for more information about IPN.

Note : This class requires cURL PHP module.

So... how should I use this ? This requires that:
a) you already know what IPN is
b) you setup your Paypal account so that IPN are sent to a URL on your website
In this example, let's consider it is http://yourserver.com/backoffice/ipn.php

1) Paypal server posts a request to http://yourserver.com/backoffice/ipn.php
2) Create the IPN object: $pp = new paypal_ipn ();
3) Provide the IPN object with 2 functions: the first one will be called as soon as you receive the post. The second one will be called when Paypal's server
confirms or denies your response.
$pp->set_process_functions ("my_pre_processing", "my_post_processing");
4) Pass the POST table to the Paypal processor
$pp->process_notification ($_POST, true);
5) The "my_pre_processing" function is automatically called
6) The class calls-back Paypal secure server, using cURL and Paypal server
confirms or denies your response.
7) The "my_post_processing" function is automatically called