How can I add an all option to a dropdown to query all categories in yii2?

So I have created a simple search form, and created the query within my controller (I am aware this isn’t the best way to do it and should be done in my model/search model however for the time being it will do).

I was wondering how can I add an option to my form within the two dropdown with the label All which if passed will mean that that part of the query isn’t applied.

Below is my view

      <?php $form = ActiveForm::begin(['id' => 'home-search','method' => 'post', 'action' => Url::to(['productitem/search'])]); ?>

    <?= $form->field($productitem, 'name')->textInput(array('placeholder' => 'What are you looking for?'))->label(false) ?>

    <?= $form->field($productitem, 'brand_id')->dropDownList(


        ['prompt'=>'Select Brand']

    )->label(false) ?>

    <?= $form->field($productitem, 'category_id')->dropDownList(


        ['prompt'=>'Select Department']

    )->label(false) ?>

    <div class="form-group search-button">

        <button type="submit" class="btn btn-primary" name="login-button">Search <i class="fa fa-lg fa-arrow-circle-o-right"></i></button>


    <?php ActiveForm::end(); ?>

Below is my controller/query

    public function actionSearch()


    $query = ProductItem::find()

    ->andFilterWhere(['like', 'name', $_POST['ProductItem']['name']])

    ->andFilterWhere(['in', 'brand_id', $_POST['ProductItem']['brand_id']])

    ->andFilterWhere(['in', 'category_id', $_POST['ProductItem']['category_id']]);

    $dataProvider = new ActiveDataProvider([

        'query' => $query


    return $this->render('search', [

        'dataProvider' => $dataProvider,