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