code: protected function renderHeader()
      {
          if ($this->header !== null) {
              $content = '';
              if($this->headerIcon !== null){
                 $content .=  Html::tag('i', '', ['class' => $this->headerIcon]);
              }
              
              $content .=  Html::tag('h3', "\n" . $this->header . "\n", ['class' => 'box-title']);
              
              if($this->headerButton !== null){
                 $content .=  Html::tag('div', $this->headerButton, ['class' => ($this->headerButtonGroup) ? 'pull-right box-tools btn-group':'pull-right box-tools']);
              }
              
              if($this->usebutton){
                  $content .= $this->renderButton(self::POSITION_HEADER);
              }
              
              return Html::tag('div', $content, array_merge(['class' => 'box-header'],  $this->headeroption));
          } else {
              return null;
          }
      }
docs: Renders the header HTML markup of the modal
  @return string the rendering result