code: public function orderCacheRule($nbCacheRule, $duration)
      {
          return json_decode(self::getClient()->orderCacheRule($this->sn, $nbCacheRule, $duration));
      }
docs: Order cache rules and return a billing order object
  
  @param int $nbCacheRule : number of backend needed
  @param int $duration : durtaion (in month)
  @return object billing order