private function requestsMatch($vary, $r1, $r2)
    {
        if ($vary) {
            foreach (explode(',', $vary) as $header) {
                $key = trim(strtolower($header));
                $v1 = isset($r1[$key]) ? $r1[$key] : null;
                $v2 = isset($r2[$key]) ? $r2[$key] : null;
                if ($v1 !== $v2) {
                    return false;
                }
            }
        }

        return true;
    }