Yii2 dropdown select optgroup

yii2-model-dropdown-select-optgroup.php

public static function getHierarchy() {
        $options = [];
         
        $parents = self::find()->where("parent_id=0")->all();
        foreach($parents as $id => $p) {
            $children = self::find()->where("parent_id=:parent_id", [":parent_id" => $p->id])->all();
            
            $child_options = [];
            
            foreach($children as $child) {
                $child_options[$child->id] = $child->name;
            }
            $options[$p->name] = $child_options;
        }
        return $options;
    }

yii2-view-dropdown-select-optgroup.php

<?=
Html::dropDownList('Post[category]', $model->category, app\models\Category::getHierarchy(), ['prompt' => 'Category'] );
?>

<?=
    $form->field($model, 'category', [
        'inputOptions' => [
            'class' => 'selectpicker '
        ]
    ]
    )->dropDownList(app\models\Category::getHierarchy(), ['prompt' => 'Category', 'class'=>'form-control required']);
?>