public static function check($lastModifiedTime = null, $isPublic = false, $options = array())
    {
        if (null !== $lastModifiedTime) {
            $options['lastModifiedTime'] = (int)$lastModifiedTime;
        }
        $options['isPublic'] = (bool)$isPublic;
        $cg = new HTTP_ConditionalGet($options);
        $cg->sendHeaders();
        if ($cg->cacheIsValid) {
            exit();
        }
    }