GridView DataColumn 如何格式化 [ 未指定版本 ]

<?= GridView::widget([
        'dataProvider' => $dataProvider,
        //'filterModel' => $searchModel,
        'columns' => [
            //['class' => 'yii\grid\SerialColumn'],
            ['attribute'=>'id','headerOptions'=>['width'=>50]],
            //'parentid',
            ['attribute'=>'groupname','label'=>'组名','format'=>['html',['str_replace',' ','mm']]],
            //'path',
            'content',
            'user_nums',
            // 'rules',

            ['class' => 'yii\grid\ActionColumn','header'=>Yii::t('app','Action')],
        ],
    ]); ?>

我想用str_replace给那列数据替换一下，怎么写这个，有谁知道？