code: protected function initDefaultButtons()
      {
          if (!isset($this->buttons['bulkpublish'])) {
              $this->buttons['bulkpublish'] = function ($url, $model) {
                  return Html::tag('li',Html::a('<span class="fa fa-leaf fa-lg"></span> '.Yii::t('yii', 'Publish'), $url, [
                      'title' => Yii::t('yii', 'Publish'),
                      'data-pjax' => '0',
                  ]));
              };
          }
          if (!isset($this->buttons['bulkunpublish'])) {
              $this->buttons['bulkunpublish'] = function ($url, $model) {
                  return Html::tag('li',Html::a('<span class="fa fa-unlink  fa-lg"></span> '.Yii::t('yii', 'Unpublish'), $url, [
                      'title' => Yii::t('yii', 'Unpublish'),
                      'data-pjax' => '0',
                  ]));
              };
          }
          if (!isset($this->buttons['bulkdelete'])) {
              $this->buttons['bulkdelete'] = function ($url, $model) {
                  return Html::tag('li',Html::a('<span class="fa fa-trash-o fa-lg"></span> '.Yii::t('yii', 'Delete'), $url, [
                      'title' => Yii::t('yii', 'Delete'),
                      'data-confirm' => Yii::t('yii', 'Are you sure you want to delete this item?'),
                      'data-method' => 'post',
                      'data-pjax' => '0',
                  ]));
              };
          }
          
          if (!isset($this->buttons['bulkrestore'])) {
              $this->buttons['bulkrestore'] = function ($url, $model) {
                  return Html::tag('li',Html::a('<span class="fa fa-undo fa-lg"></span> '.Yii::t('yii', 'Restore'), $url, [
                      'title' => Yii::t('yii', 'Restore'),
                      'data-pjax' => '0',
                  ]));
              };
          }
      }
docs: Initializes the default button rendering callbacks