public function getExternalId()
    {
        $externalIds = array_flip($this->externalIds);
        $id          = null;
        $idIndex     = null;
        foreach (['headers', 'body'] as $type) {
            if (isset($this->responseActual[$type]) && isset($this->responseActual[$type])) {
                foreach ($this->responseActual[$type] as $key => $value) {
                    $key = preg_replace('/[^a-z0-9]/', '', strtolower($key));
                    if (isset($externalIds[$key]) && (null === $idIndex || $externalIds[$key] < $idIndex)) {
                        $idIndex = $externalIds[$key];
                        $id      = $value;
                        if (0 == $idIndex) {
                            break;
                        }
                    }
                }
            }
        }

        return $id;
    }