https://github.com/paypal/ipn-code-samples
此处有一个关于PHP对接返回的demo
https://github.com/paypal/ipn-code-samples/blob/master/php/PaypalIPN.php
PayPal的授权和捕获-Authorization and capture
https://developer.paypal.com/docs/paypal-payments-standard/integration-guide/authcapture/