code: public function addMultiple($members, $flag = null, bool $plusChanged = false): int
      {
          $members = array_combine(array_map([$this, 'valToStr'], array_keys($members)), array_values($members));
  
          $ret = $this->strToVal(
              $this->redis->zadd($this->key, $members, $flag, $flag === null ? $plusChanged : $flag !== self::FLAG_ADD)
          );
  
          if (\is_int($ret)) {
              return $ret;
          }
  
          return 1; // Only one member was in $members. We don't care for its value here.
      }
docs: @link https://redis.io/commands/zadd
  
  @param array       $members     [string|int member => float score, ...]
  @param null|string $flag        See self::FLAG_* for possible flags. (ADD, UPDATE)
  @param bool        $plusChanged Only valid if $flag is null. Adds number of changed members to result.
  Automatically true if $flag is self::FLAG_UPDATE.
  
  @return int Number of members added to set plus optionally number of changed members.