code: public function score($member)
      {
          $ret = $this->redis->zscore($this->key, $this->valToStr($member));
          if ($ret === null) {
              return null;
          }
  
          return $this->strToVal($ret);
      }
docs: @link https://redis.io/commands/zscore
  
  @param string|number $member
  
  @return null|number|string