ValidateUserNameByAjax

public function actionSignupForm()
{
	
	$model = new SignupForm();

	if (Yii::$app->request->isAjax && $model->load(Yii::$app->request->post())) {
		Yii::$app->response->format = Response::FORMAT_JSON;
		return ActiveForm::validate($model);
	}

	if ($user = $model->signup()) {
		if(Yii::$app->getUser()->login($user)) {
			return $this->goHome();
		}
	}

	return $this->render('signup', [
		'model' => $model,
 	]);

}