kartik\grid\BooleanColumn::getDataCellValue PHP Method

    public function getDataCellValue($model, $key, $index)
    {
        $value = parent::getDataCellValue($model, $key, $index);
        if ($value !== null) {
            return $value ? $this->trueIcon : $this->falseIcon;
        }
        return $this->showNullAsFalse ? $this->falseIcon : $value;
    }