code: public function begin()
      {
          if ($this->form->enableClientScript) {
              $clientOptions = $this->getClientOptions();
              if (!empty($clientOptions)) {
                  $this->form->attributes[] = $clientOptions;
              }
          }
  
          $inputID = $this->getInputId();
          $attribute = Html::getAttributeName($this->attribute);
          $options = $this->options;
          $class = isset($options['class']) ? (array) $options['class'] : [];
          $class[] = "field-$inputID";
          if ($this->model->isAttributeRequired($attribute)) {
              $class[] = $this->form->requiredCssClass;
          }
          $options['class'] = implode(' ', $class);
          if ($this->form->validationStateOn === ActiveForm::VALIDATION_STATE_ON_CONTAINER) {
              $this->addErrorClassIfNeeded($options);
          }
          $tag = ArrayHelper::remove($options, 'tag', 'div');
  
          return Html::beginTag($tag, $options);
      }
docs: Renders the opening tag of the field container.
  @return string the rendering result.