private function _getServerBaseURL()
    {
        // the URL is build only when needed
        if ( empty($this->_server['base_url']) ) {
            $this->_server['base_url'] = 'https://' . $this->_getServerHostname();
            if ($this->_getServerPort()!=443) {
                $this->_server['base_url'] .= ':'
                .$this->_getServerPort();
            }
            $this->_server['base_url'] .= $this->_getServerURI();
        }
        return $this->_server['base_url'];
    }