private function getOauthHeaders($includeCallback = false)
    {
        $oauth = array(
            'oauth_consumer_key'     => $this->_client_id,
            'oauth_nonce'            => $this->generateNonce(),
            'oauth_signature_method' => 'HMAC-SHA1',
            'oauth_timestamp'        => time(),
            'oauth_version'          => '1.0'
        );

        if (isset($this->_access_token)) {
            $oauth['oauth_token'] = $this->_access_token;
        } elseif (isset($_SESSION['oauth']['token'])) {
            $oauth['oauth_token'] = $_SESSION['oauth']['token'];
        }

        if ($includeCallback) {
            $oauth['oauth_callback'] = $this->_callback;
        }

        return $oauth;
    }