protected function checkCache($originalFile, $cacheFile)
    {
        if (! $ret = parent::checkCache($originalFile, $cacheFile)) {
            return false;
        }

        if (Utils::isSerialized($ret, $ret)) {
            // Go through each file and make sure none of them have changed
            foreach ($ret['files'] as $file => $lastModified) {
                if (filemtime($file) > $lastModified) {
                    return false;
                }
            }

            $ret = serialize($ret);
        }

        return $ret;
    }