Yii2 displaying a menu with an unlimited nesting level from Nested Sets

menu.php

<?php

/**
 * @var $this yii\web\View
 * @var $categories \app\models\Category[]
 * @var $level_start int
 */

use yii\helpers\Html;

/**
 * Displaying a menu with an unlimited nesting level from Nested Sets
 * ported code for yii2 from https://github.com/yiiext/nested-set-behavior/blob/master/readme_ru.md#%D0%9E%D0%B1%D1%85%D0%BE%D0%B4-%D0%B4%D0%B5%D1%80%D0%B5%D0%B2%D0%B0-%D0%B1%D0%B5%D0%B7-%D1%80%D0%B5%D0%BA%D1%83%D1%80%D1%81%D0%B8%D0%B8
 *
 * example:
 * 
 * $root_node = Category::find()->where(['depth' => 0])->one();
 * $level_start = $root_node->depth;
 * $categories = $root_node->children()->all();
 * return $this->render('sidebar', ['categories' => $categories, 'level_start' => $level_start]);
 * 
 */

$level = $level_start;
$menu = null;

foreach ($categories as $key => $category) {
    switch ($category->depth) {
        case ($category->depth == $level):
            $menu .= Html::endTag('li') . PHP_EOL;
            break;
        case $category->depth > $level:
            $class_ul = $level == $level_start ? 'main-menu' : 'sub-menu';
            $menu .= Html::beginTag('ul', ['class' => $class_ul]);
            break;
        case $category->depth < $level:
            $menu .= Html::endTag('li') . PHP_EOL;
            for ($i = $level - $category->depth; $i; $i--) {
                $menu .= Html::endTag('ul') . PHP_EOL;
                $menu .= Html::endTag('li') . PHP_EOL;
            }
            break;
    };

    //If you do not need to add a class to the parent tag li
    $menu .= Html::beginTag('li');
    $menu .= Html::a($category->name, '#');

    //if you want to add a class to the parent tag li uncomment this block && comment prev block
    /*if (isset($categories[$key + 1]) && $categories[$key + 1]->depth > $category->depth) {
        $menu .= Html::beginTag('li', ['class' => 'parent']);
        $menu .= Html::a($category->name, '/category');
    } else {
        $menu .= Html::beginTag('li');
        $menu .= Html::a($category->name, '/sub-category');
    }*/

    $level = $category->depth;
}

for ($i = $level; $i > $level_start; $i--) {
    $menu .= Html::endTag('li') . PHP_EOL;
    $menu .= Html::endTag('ul') . PHP_EOL;
}

echo $menu;