code: public function add($member, $score, $flag = null, $incr = false)
      {
          $ret = $this->strToVal(
              $this->redis->zadd(
                  $this->key,
                  [$this->valToStr($member) => $score],
                  $flag,
                  $flag === self::FLAG_UPDATE,
                  $incr
              )
          );
          if ($incr) {
              return $ret;
          }
  
          return $ret === 1;
      }
docs: @link https://redis.io/commands/zadd
  
  @param string|int  $member Member to add or update.
  @param float|int   $score  Associated score to member.
  @param null|string $flag   See self::FLAG_* for possible flags. (ADD, UPDATE)
  @param bool        $incr   True will make add() behave like increment($member, $score).
  
  @return number|string|bool Bool on setting or updating to report the success or the resulting value when using
  $incr.