code: protected function addMemcachedServers($cache, $servers)
      {
          $existingServers = [];
          if ($this->persistentId !== null) {
              foreach ($cache->getServerList() as $s) {
                  $existingServers[$s['host'] . ':' . $s['port']] = true;
              }
          }
          foreach ($servers as $server) {
              if (empty($existingServers) || !isset($existingServers[$server->host . ':' . $server->port])) {
                  $cache->addServer($server->host, $server->port, $server->weight);
              }
          }
      }
docs: Add servers to the server pool of the cache specified
  Used for memcached PECL extension.
  
  @param \Memcached $cache
  @param MemCacheServer[] $servers