public static function getCanonical()
    {
        if (!isset($_SERVER['REQUEST_METHOD'])) {
            // ALERT - no current URL
            throw new Exception('Script was not called through a webserver');
        }

        // Begin with a relative URL
        $url = new self($_SERVER['PHP_SELF']);
        $url->_scheme = isset($_SERVER['HTTPS']) ? 'https' : 'http';
        $url->_host   = $_SERVER['SERVER_NAME'];
        $port = $_SERVER['SERVER_PORT'];
        if ($url->_scheme == 'http' && $port != 80
            || $url->_scheme == 'https' && $port != 443
        ) {
            $url->_port = $port;
        }
        return $url;
    }