code: public static function validate($model, $attributes = null)
      {
          $result = [];
          if ($attributes instanceof Model) {
              // validating multiple models
              $models = func_get_args();
              $attributes = null;
          } else {
              $models = [$model];
          }
          /* @var $model Model */
          foreach ($models as $model) {
              $model->validate($attributes);
              foreach ($model->getErrors() as $attribute => $errors) {
                  $result[Html::getInputId($model, $attribute)] = $errors;
              }
          }
  
          return $result;
      }
docs: Validates one or several models and returns an error message array indexed by the attribute IDs.
  This is a helper method that simplifies the way of writing AJAX validation code.
  
  For example, you may use the following code in a controller action to respond
  to an AJAX validation request:
  
  ```php
  $model = new Post;
  $model->load(Yii::$app->request->post());
  if (Yii::$app->request->isAjax) {
  Yii::$app->response->format = Response::FORMAT_JSON;
  return ActiveForm::validate($model);
  }
  // ... respond to non-AJAX request ...
  ```
  
  To validate multiple models, simply pass each model as a parameter to this method, like
  the following:
  
  ```php
  ActiveForm::validate($model1, $model2, ...);
  ```
  
  @param Model $model the model to be validated.
  @param mixed $attributes list of attributes that should be validated.
  If this parameter is empty, it means any attribute listed in the applicable
  validation rules should be validated.
  
  When this method is used to validate multiple models, this parameter will be interpreted
  as a model.
  
  @return array the error message array indexed by the attribute IDs.