When developing an API callback endpoint in PHP, the following sample code may be used for debugging purposes:

<?

$message = print_r($_REQUEST, true);
$message .= 'SMSID:' . $smsid.';';
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.