IPN Third party integration for S2Member plugin

Integrating IPN script to S2Member wordpress plugin for membership management. Redirection from payment checkout to registration form page for paying members required. There is an existing script which needs to be adjusted. Please view attached php script (the one to be adjusted) and documentation of S2Member API. These requirements are existing: "If you're using a 3rd-party application that needs to POST simulated IPN transactions to your s2Member installation, you can use this alternate IPN URL, which includes a Proxy Key. This encrypted Proxy Key verifies incoming data being received by s2Member's IPN processor. You can change [proxy-gateway] to whatever you like. The [proxy-gateway] value is required. It will be stored by s2Member as the Customer's Paid Subscr. Gateway. Your [proxy-gateway] value will also be reflected in s2Member's IPN log. [[login to view URL][proxy-gateway]&s2member_paypal_proxy_verification=7410e47474d2b5411f87b5c31ac33d4e] Any 3rd-party application that is sending IPN transactions to your s2Member installation, must ALWAYS include the custom POST variable, and that variable must always start with your installation domain ( i.e. custom=[login to view URL] ). In addition, the item_number variable, must always match a format that s2Member looks for. Generally speaking, the item_number should be 1, 2, 3, or 4, indicating a specific s2Member Level #. However, s2Member also uses some advanced formats in this field. Just to be sure, we suggest creating a PayPal® Button with the s2Member Button Generator, and then taking a look at the Full Button Code to see how s2Member expects item_number to be formatted. Other than the aforementioned exceptions; all other POST variables should follow PayPal® standards. Please see: PayPal's IPN/PDT reference guide for full documentation." Remark: I do NOT use PayPal, but script would have to work like the paypal button, I think. For further information, please visit [login to view URL] Thank you!