Categories followed by their subcategories in dropdown – Yii2.0

For Example

Parent1
__Child1-Parent1
__Child2-Parent1
__|__Child1-Child2-Parent1
Parent2
__Child1-Parent2
__Child2-Parent2
__|__Child1-Child2-Parent2
Parent3
Parent4
__Child-parent4

Create category table.

CREATE TABLE IF NOT EXISTS `category` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
`slug` varchar(255) NOT NULL,
`description` text NOT NULL,
`image` varchar(250) NOT NULL,
`order` int(11) NOT NULL,
`parent_id` int(11) NOT NULL,
`status` int(11) NOT NULL DEFAULT '0' COMMENT '0=> inactive, 1=>active',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;

Open models/Category.php. Then add below code,

public function getCategories($parent_id = 0, $exclude = '', $space = '', $categories='')
{
if($parent_id == 0){
$space = '';
$categories = array();
}else{
$space .='- ';
}

$model = Category::findAll([
'parent_id' => $parent_id,
]);
if(!empty($model)){
foreach ($model as $key) {
if($key->id == $exclude) continue;
$categories[] = array('id'=>$key->id, 'name'=>$space.$key->name);
$categories = $this->getCategories($key->id, $exclude, $space, $categories);
}
}

return $categories;
}

Open views/category/_form.php

<?php

use yii\helpers\Html;
use yii\widgets\ActiveForm;
use yii\helpers\ArrayHelper;
use app\models\Category;
?>

<?php 
 $category = new Category;
 $categories = $category->getCategories(0, $model->id);
 $items = ArrayHelper::map($categories, 'id', 'name');
?>
<?php echo $form->field($model, 'parent_id')->dropDownList($items,['prompt'=>'--None--']); ?>

 'btn btn-success']) ?>

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