function set_base_url($url = '')
    {
        if (empty($url)) {
            if (!empty($_SERVER['HTTP_HOST'])) {
                $this->url = 'http://' . $_SERVER['HTTP_HOST'];
            }
            else {
                $this->url = '';
            }
        }
        else {
            // Strip any trailing slashes for consistency (relative
            // URLs may already start with a slash like "/file.html")
            if (substr($url, -1) == '/') {
                $url = substr($url, 0, -1);
            }
            $this->url = $url;
        }
    }