<?= $form->field($model, 'maincatalog')->dropDownList($maincatalog, ["prompt" => Yii::t('app', 'select_cat')])->label(false) ?>

<?= $form->field($model, 'listproduct')->dropDownList($listproduct, ["prompt" => Yii::t('app', 'select_cat')])->label(false) ?>

<?= $form->field($model, 'q')->textInput(['class' => 'input'])->label('')?>


$query = Addproduct::find()
                     ->where(['catalog' => $maincatalog])
                     ->andWhere(['product' => $listproduct])
                     ->andWhere(['like', 'titleru', $q])->all();

$query = Addproduct::find();

    if ( $maincatalog != NULL )
        $query->where(['catalog' => $maincatalog]);


    $query->andWhere(['product' => $listproduct])
          ->andWhere(['like', 'titleru', $q])->all();