code: protected function renderHeader()
      {
          if ($this->header !== null) {
              $content = Html::tag('h3', $this->header);
              
              if($this->caption !== NULL){
                  $content .= Html::tag('p', $this->caption);
              }
              return Html::tag('div', $content, ['class' => 'inner']);
          } else {
              return null;
          }
      }
docs: Renders the header HTML markup of the modal
  @return string the rendering result