yii\grid\SerialColumn::renderDataCellContent PHP Method

    protected function renderDataCellContent($model, $key, $index)
    {
        $pagination = $this->grid->dataProvider->getPagination();
        if ($pagination !== false) {
            return $pagination->getOffset() + $index + 1;
        } else {
            return $index + 1;
        }
    }