RequestToken.php

 /**
     * Request OAuth access token
     *
     * @param  string $code OAuth code we got from service
     * @return string Token, or null if we didn't obtain a proper token
     */
    protected function _requestToken($code)
    {
        $config = $this->_config->oauth;

        $client = new \Zend_Http_Client("http://accounts.tunehog.com/oauth/token");
        $client->setParameterPost([
            'grant_type' => 'authorization_code',
            'client_id' => $config->client_id,
            'client_secret' => $config->client_secret,
            'code' => $code,
        ]);

        if ($response = $this->_helper->http->request($client, 'POST')) {
            $response = \Zend_Json::decode($response->getBody(), \Zend_Json::TYPE_OBJECT);
            if (isset($response->access_token)) {
                return $response->access_token;
            }
        }
    }