private function requestsMatch($vary, $env1, $env2)
    {
        if (empty($vary)) {
            return true;
        }

        foreach (preg_split('/[\s,]+/', $vary) as $header) {
            $key = str_replace('_', '-', strtolower($header));
            $v1 = isset($env1[$key]) ? $env1[$key] : null;
            $v2 = isset($env2[$key]) ? $env2[$key] : null;
            if ($v1 !== $v2) {
                return false;
            }
        }

        return true;
    }