When developing an API callback endpoint in PHP, the following sample code may be used for debugging purposes. When a callback is received it will be sent to the nominated e-mail address as a print_r output.
$message = "Callback response received: \n\n" . print_r($_REQUEST, true);
mail('Your@Email.Address', 'API Callback Debug', $message);
This will send you an e-mail containing all the values passed to your callback endpoint from the API server.
If you don't receive the e-mail, please check your spam filter, or /var/log/mail.log on the server running your code.