You're seeing a blank page because you don't send any POST data. The reply to bitcoinnotify's request would contain the POST data in its reply.
If you need to see the data, your script needs to log it. You can simply dump it into a file:
fwrite($f, "[".date('r')."]: data received\n");
fwrite($f, ': POST: '.print_r($_POST, true));
Remember that, depending on your config, you might have to adjust the permissions on the log file so that your webserver can write to it.