<?php 
$form = ActiveForm::begin();
?>

    <?php 
echo $form->field($model, 'the_must')->textarea(['rows' => 6]);
?>

    <?php 
echo $form->field($model, 'the_best')->textarea(['rows' => 6]);
?>

  <?php 
//$form->field($model, 'group_id')->dropdownList(Group::getHierarchy())
?>
    <?php 
echo $form->field($model, 'group_id')->widget(Select2::classname(), ['data' => Group::getHierarchy(), 'options' => ['placeholder' => 'เลือกหัวข้อ ...'], 'pluginOptions' => ['allowClear' => true]]);
?>
    <div class="form-group">
        <?php 
echo Html::submitButton($model->isNewRecord ? Yii::t('app', 'Create') : Yii::t('app', 'Update'), ['class' => $model->isNewRecord ? 'btn btn-success' : 'btn btn-primary']);
?>
    </div>

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

</div>
use yii\web\JsExpression;
/* @var $this yii\web\View */
/* @var $model common\models\Group */
/* @var $form yii\widgets\ActiveForm */
?>

<div class="group-form">

    <?php 
$form = ActiveForm::begin();
?>

    <?php 
echo $form->field($model, 'name')->textInput(['maxlength' => true]);
?>
    <?php 
echo $form->field($model, 'parent_id')->dropdownList(ArrayHelper::merge([null => 'None'], Group::getHierarchy()));
?>

    <div class="form-group">
        <?php 
echo Html::submitButton($model->isNewRecord ? Yii::t('app', 'Create') : Yii::t('app', 'Update'), ['class' => $model->isNewRecord ? 'btn btn-success' : 'btn btn-primary']);
?>
    </div>

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

</div>