private function getCompositeKey()
    {
        if (isset($this->_access_token_secret) && strlen($this->_access_token_secret) > 0) {
            $composite_key = $this->encode($this->_client_secret).'&'.$this->encode($this->_access_token_secret);
        } elseif (isset($_SESSION['oauth']['token_secret'])) {
            $composite_key = $this->encode($this->_client_secret).'&'.$this->encode($_SESSION['oauth']['token_secret']);
        } else {
            $composite_key = $this->encode($this->_client_secret).'&';
        }

        return $composite_key;
    }