framework/grid/CheckboxColumn.php
protected function renderHeaderCellContent()
    {
        if ($this->header !== null || !$this->multiple) {
            return parent::renderHeaderCellContent();
        }

        return Html::checkbox($this->getHeaderCheckBoxName(), false, ['class' => 'select-on-check-all']);
    }
protected function renderHeaderCellContent()
    {
        if ($this->header !== null || !$this->multiple) {
            return parent::renderHeaderCellContent();
        }

        return Html::checkbox($this->getHeaderCheckBoxName(), false, ['class' => 'select-on-check-all']);
    }
Renders the header cell content.
The default implementation simply renders [[header]].
This method may be overridden to customize the rendering of the header cell.
@return string the rendering result