public function queryVar(string $key): string
    {
        if (isset($this->query_vars[$key])) {
            return $this->query_vars[$key];
        }
        $value = ''; // Initialize.

        if (isset($_REQUEST[$key])) {
            $value = (string) $_REQUEST[$key];
            $value = $this->c::unslash($value);
            $value = $this->c::mbTrim($value);
        } elseif (isset($this->rewrite_query_vars[$key])) {
            $value = $this->rewrite_query_vars[$key];
        }
        return $this->query_vars[$key] = $value;
    }