code: public function postInstanceReboot($serviceName, $idInstance, $type='soft'){
          if (!$serviceName)
              throw new BadMethodCallException('Missing parameter $serviceName (OVH Cloud Project).');
          if (!$idInstance)
              throw new BadMethodCallException('Missing parameter $idInstance (OVH Cloud Instance ID).');           
          
          $post = array(
              'type' => $type
  		);
           
          try {
              $r = $this->post('cloud/project/'.$serviceName.'/instance/'.$idInstance.'/reboot', array('Content-Type' => 'application/json;charset=UTF-8'), json_encode($post))->send();
          }
          catch (\Exception $e) {
              throw new CloudException($e->getMessage(), $e->getCode(), $e);
          }
          return $r->getBody(true);
      }
docs: POST Reboot Instance
  POST /cloud/project/{serviceName}/instance/{instanceId}/reboot
  
  @return null
  @throws \Ovh\Cloud\Exception\CloudException
  @throws \Ovh\Common\Exception\BadMethodCallException