PayPal PDT and no transaction token returned

Question

I'm trying to get pdt data back from paypal sandbox, seems pretty straightforward but I don't get any data actually. I'm able to call the service and get the callback but there is no data.

I'm following this example: https://github.com/paypal/pdt-code-samples/blob/master/paypal_pdt.php

and also did set my selling tools settings specifying:

• Auto return: on
• Payment Data Transfer: on

The main issue is the transaction token, the line below has no value

$tx_token = $_GET['tx'];

Do I need some specific configuration on my server or what else I'm missing here? Should I use IPN?

Thanks