Paypal SandBox IPN always returns INVALID

Question

As mentioned in one of the comments in an answer below, I tried following this tutorial. So now I have the following:

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The ipn.php file:

Answer 1

1. Got it working using the basic sample code 4b,

2. Cleared $ipnNotificationUrl = ""; from the basic sample code as I had a value in there which I added myself,

3. Created a seller account instead of a business pro account in sandbox,

4. Set the seller account to enable the ipn url,

5. Used the following PHP 5.2 sample code for the ipn listener

6. Added the 2 lines into the listener, as described here, the 2 lines can be seen below:

7. Downloaded the cacert.pem certificate to my server from here and put it in the same directory as the ipn listener:

The 2 lines mentioned in point 6:

CURLOPT_SSL_VERIFYPEER => TRUE,
CURLOPT_CAINFO => 'cacert.pem',

I have no idea why the sandbox business pro account does not let me set an ipn url, but the seller account does.