Amazon MWS API 提供了 cancelOrder 接口来取消订单。使用该接口需要具有订单管理权限。以下是示例代码：

function cancelOrder($orderId) {

  $config = array (
    'ServiceURL' => "https://mws.amazonservices.com/Orders/2013-09-01",
    'ProxyHost' => null,
    'ProxyPort' => -1,
    'MaxErrorRetry' => 3,
  );

  $service = new MarketplaceWebServiceOrders_Client(
    AWS_ACCESS_KEY_ID,
    AWS_SECRET_ACCESS_KEY,
    APPLICATION_NAME,
    APPLICATION_VERSION,
    $config);

  $request = new MarketplaceWebServiceOrders_Model_CancelOrderRequest();
  $request->setSellerId(MERCHANT_ID);
  $request->setAmazonOrderId($orderId);

  return $service->cancelOrder($request);

}

// 示例用法
$orderId = "123-1234567-1234567";
$result = cancelOrder($orderId);

需要将 AWS_ACCESS_KEY_ID，AWS_SECRET_ACCESS_KEY 和 MERCHANT_ID 替换为您自己的值。