public function getAuthorizationHeader($headers, $url, $queryParams, $httpMethod)
    {
        if (($this->accessToken == null)
            || ($this->accessToken->getExpiresIn() < time())
        ) {
            $this->accessToken = $this->oauthService->getAccessToken(
                $this->grantType,
                $this->accountName,
                $this->accountKey,
                $this->scope
            );
        }

        return Resources::OAUTH_ACCESS_TOKEN_PREFIX.
            $this->accessToken->getAccessToken();
    }