code: public function actionRegister()
  	{
  		$model = new User();
  		$model->scenario = 'register';
  
  		if ($model->load(Yii::$app->request->post()) && $model->register(FALSE, User::STATUS_PENDING)) {
  			// Send Welcome Message to activate the account
  			Mailer::sendWelcomeMessage($model);
  
  			Yii::$app->session->setFlash(
  				'success', Yii::t('user', 'You\'ve successfully been registered. Check your mail to activate your account'));
  
  			return $this->redirect(Yii::$app->urlManager->createUrl('//user/auth/login'));
  		}
  
  		return $this->render('register', ['model' => $model]);
  	}
docs: Register the user
  
  @return string|\yii\web\Response