Smopp_v2/tmc/ajax/get_stacks.php

<?php /** Created by Anton on 31.01.2020. */

require '../yiiInit.php';

$warehouseId = Yii::$app->request->get('warehouse');
$warehouse = \app\models\entity\Warehouse::findOne($warehouseId);
$stacks = $warehouse->getStacks();
foreach ($stacks as $stack): ?>
    <option value="<?= $stack->id ?>"><?= $stack->title ?></option>
<?php endforeach; ?>