code: public function rank($member, bool $reverse = false): ?int
      {
          if ($reverse) {
              return $this->redis->zrevrank($this->key, $this->valToStr($member));
          }
  
          return $this->redis->zrank($this->key, $this->valToStr($member));
      }
docs: @link https://redis.io/commands/zrank
  @link https://redis.io/commands/zrevrank
  
  @param string|number $member
  
  @param bool          $reverse
  
  @return int|null