code: protected function addServers($cache, $servers)
      {
          if (empty($servers)) {
              $servers = [new MemCacheServer([
                  'host' => '127.0.0.1',
                  'port' => 11211,
              ])];
          } else {
              foreach ($servers as $server) {
                  if ($server->host === null) {
                      throw new InvalidConfigException("The 'host' property must be specified for every memcache server.");
                  }
              }
          }
          if ($this->useMemcached) {
              $this->addMemcachedServers($cache, $servers);
          } else {
              $this->addMemcacheServers($cache, $servers);
          }
      }
docs: Add servers to the server pool of the cache specified.
  
  @param \Memcache|\Memcached $cache
  @param MemCacheServer[] $servers
  @throws InvalidConfigException