PHP PayPal IPN Solution

PHP PayPal IPN
How exactly is this PHP paypal IPN used?
I found a lot of code online, someone wrote a good class, directly can call on it, the bottom of the reply is also can be used.
Why can't I use it? I've been testing in the sandbox.

is not in the Sandbox test tool point send IPN, and then open their own server ipn.php on the test?
Or do I have a wrong test method in the sandbox?

Who used IPN, teach me how to use AH

------Solution--------------------
IPN is essentially a URL callback function, and the method you test is right.
It should be noted that the URL port of the callback must be 80

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