Yii2 NestedSets class trait as a nice helper to use with any class that uses the NestedSets behavior

NestedSetToArrayExample.php

<?php
/**
 * NestedSets in SQL - How to export a nested set subtree or tree into Yii2 Menu widget and as
 * JsTree valid DataSet (nested format).
 */
class Category extends \yii\db\ActiveRecord
{
    /**
     * @return array
     */
    public function behaviors()
    {
        $rVal = parent::behaviors();
        $rVal[] = [
            // @link https://github.com/creocoder/yii2-nested-sets
            'class' => NestedSetsBehavior::className(),
        ];

        return $rVal;
    }

    /**
     * @return array
     */
    public function transactions()
    {
        return [
            self::SCENARIO_DEFAULT => self::OP_ALL,
        ];
    }
    
    /**
     * Convert a tree into nested arrays. If you use the default function parameters you get
     * a set compatible with Yii2 Menu widget.
     *
     * @param int $depth
     * @param string $itemsKey
     * @param callable|null $getDataCallback
     * @return array
     */
    public function toNestedArray($depth = null, $itemsKey = 'items', $getDataCallback = null)
    {
        /** @var Category $nodes */
        $nodes = $this->children($depth)->all();
        $exportedAttributes = array_diff(array_keys($this->attributes), ['lft', 'rgt']);
    
        $trees = [];
        $stack = [];
    
        foreach ($nodes as $node) {
            if ($getDataCallback) {
                $item = call_user_func($getDataCallback, $node);
            } else {
                $item = $node->toArray($exportedAttributes);
                $item['url'] = [UrlRuleCategory::ROUTE, 'slug' => $item['slug']];
            }
    
            $item[$itemsKey] = [];
            $l = count($stack);
    
            while ($l > 0 && $stack[$l - 1]['depth'] >= $item['depth']) {
                array_pop($stack);
                $l--;
            }
    
            if ($l == 0) {
                // Assign root node
                $i = count($trees);
                $trees[$i] = $item;
                $stack[] = &$trees[$i];
            } else {
                // Add node to parent
                $i = count($stack[$l - 1][$itemsKey]);
                $stack[$l - 1][$itemsKey][$i] = $item;
                $stack[] = &$stack[$l - 1][$itemsKey][$i];
            }
        }
    
        return $trees;
    }
    
    
    /**
     * Export NestedSets tree into JsTree nested format data
     * @return array
     */
    public static function asJsTree()
    {
        $rVal = [];
    
        /** @var Category[] $roots */
        $roots = static::find()->roots()->all();
        foreach ($roots as $root) {
            $rVal[] = [
                'id' => $root->id,
                'text' => $root->label,
                'children' => $root->toNestedArray(null, 'children', function($node) {
                    return [
                        'id' => $node->id,
                        'text' => $node->label,
                    ];
                }),
            ];
        }
    
        return $rVal;
    }
}

Thanks for this example.
Undefined index: depth in this line: while ($l > 0 && $stack[$l - 1]['depth'] >= $item['depth']) {
add depth in asJsTree():

...
                'children' => $root->toNestedArray(null, 'children', function($node) {
                    return [
                        'id' => $node->id,
                        'text' => $node->label,
                        'depth' => $node->depth,//add
                    ];
                }),
...