// Your Merchant Key
$server_type = "sandbox";
// change this to go live
$currency = 'USD';
// set to GBP if in the UK
$Gresponse = new GoogleResponse($merchant_id, $merchant_key);
$Grequest = new GoogleRequest($merchant_id, $merchant_key, $server_type, $currency);
//serial-number
//Setup the log file
$Gresponse->SetLogFiles(RESPONSE_HANDLER_ERROR_LOG_FILE, RESPONSE_HANDLER_LOG_FILE, L_ALL);
$Grequest->SetLogFiles(RESPONSE_HANDLER_ERROR_LOG_FILE, RESPONSE_HANDLER_LOG_FILE, L_ALL);
//How to grab the serial number!!!!
$serial = $_POST["serial-number"];
$Gresponse->log->LogResponse("serial:" . $serial);
if ($serial != null) {
    $Grequest->SendNotificationRequest($serial);
    $Gresponse->SendAck($serial);
    return;
}
// Retrieve the XML sent in the HTTP POST request to the ResponseHandler
$xml_response = isset($HTTP_RAW_POST_DATA) ? $HTTP_RAW_POST_DATA : file_get_contents("php://input");
if (get_magic_quotes_gpc()) {
    $xml_response = stripslashes($xml_response);
}
list($root, $data) = $Gresponse->GetParsedXML($xml_response);
$Gresponse->SetMerchantAuthentication($merchant_id, $merchant_key);
/*$status = $Gresponse->HttpAuthentication();
  if(! $status) {
    die('authentication failed');
  
  }*/