I need an IPN script for my wp woocommerce site to configure with paypal.

I need pay pal to redirect back to my order form (Gravity form embedded in a word press page) Iv tryed many times in the sandbox accounts but always comes back to customer account page showing there order details.

I need the php file called am IPN instant payment notification) and all this to be configured and working in my sandbox so i can swap paypal sandbox details for real pay pal account.

using latest wp: Customizer theme, Woocommerce, with adons for GravityForms & subscriptions. pay-pal business acc