public function getServerProxyURL()
    {
        // the URL is build only when needed
        if ( empty($this->_server['proxy_url']) ) {
            switch ($this->getServerVersion()) {
            case CAS_VERSION_1_0:
                $this->_server['proxy_url'] = '';
                break;
            case CAS_VERSION_2_0:
            case CAS_VERSION_3_0:
                $this->_server['proxy_url'] = $this->_getServerBaseURL().'proxy';
                break;
            }
        }
        return $this->_server['proxy_url'];
    }