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What is IPN?

How does it work?
  1. After finishing the payment on PayPal, the customer is auto-redirected to your page (“return” variable)
  2. Customer returns to your page. PayPal does NOT send any payment data there.
  3. Separately in the background, you receive a form POST from PayPal at a different URL (notify_url variable).
  4. You post back a form with cmd=_notify-validate and all fields you received from PayPal. PayPal responds with a single word VERIFIED or INVALID
  5. If you receive VERIFIED, you can be confident that the form you received came from PayPal and wasn’t tampered with. Do whatever you need to do with the form fields.
Settings:
  • Specify an auto return url in your profile or in the return variable in your html form. The url must be an absolute url. This is just a generic page with no PayPal processing logic. Display something like “Thank you and your order will be processed shortly.” Code: 
    <input type="hidden" name="return" value="your_return_url_here">
  • Specify an IPN url in your profile or in the notify_url variable in your html form. This is where you process payment data from PayPal. The IPN url must be an absolute url. It must also allow anonymous access from outside of your network. If you must open your firewall to a specific host, please note the Sandbox sends IPNs from ipn.sandbox.paypal.com. PayPal live site sends IPNs from notify.paypal.com. Code: 
    <input type="hidden" name="notify_url" value="your_ipn_url_here">
  • Auto Return = Enabled in account profile
  • PDT = Disabled in account profile
  • IPN = Enabled in account profile



What does EGPCS stand for? In PHP

Environment, Get, Post, Cookie, Server (predefined variables in PHP)

This directive describes the order in which PHP registers GET, POST, Cookie, Environment and Built-in variables (G, P, C, E & S respectively, often referred to as EGPCS or GPC).  Registration is done from left to right, newer values override older values.
default order of it is:

variables_order = “EGPCS”