Merge two fields to activeDropDownList in Grid column

Hello all,

in grid of my index page i want a DropDownList to filter data.

In my DropDownList i wanted to get the firstName and Surname to be returned as one string.

My code not work well. The activeDropDownList is empty. Why ?

This is my code:

public function getFullName()


return $this->first_name . ' ' . $this->first_name;


<?= GridView::widget([

    'dataProvider' =&gt; &#036;dataProvider,

    'filterModel' =&gt; &#036;searchModel,

    'columns' =&gt; [

        ['class' =&gt; 'yii&#092;grid&#092;SerialColumn'],





        'attribute' =&gt; 'User',

        'value' =&gt; 'id_user',

        'filter' =&gt; Html::activeDropDownList(&#036;searchModel, 'id_user', ArrayHelper::map(&#092;app&#092;models&#092;user::find()-&gt;asArray()-&gt;all(), 'id_user','fullName'),['class'=&gt;'form-control','prompt' =&gt; '---- Seleziona ----']),



        ['class' =&gt; 'yii&#092;grid&#092;ActionColumn'],


]); ?&gt;

What displays