public function canonicalUrl($url)
    {
        if (strlen($url)) {
            if ($url[0] == '/') {
                // If the URL begins with a "/", the following is the
                // canonical URL
                return substr($url, 1);
            } else {
                // Else, the canonical name is under the current dir
                if ($this->getDirName()) {
                    return $this->canonicalize($this->getDirName() . '/' .$url);
                } else {
                    return $this->canonicalize($url);
                }
            }
        }

        return null;
    }