think\cache\driver\Memcached::clear PHP Method

    public function clear($tag = null)
    {
        if ($tag) {
            // 指定标签清除
            $keys = $this->getTagItem($tag);
            $this->handler->deleteMulti($keys);
            $this->rm('tag_' . md5($tag));
            return true;
        }
        return $this->handler->flush();
    }