tree.php

interface HasLevel {
    /**
     * @return int
     */
    public function getLevel();
}

class Node implements HasLevel {
    public $items = [];
    public $level = 0;
    public $childNode;

    public function getLevel()
    {
        return $this->level;
    }
}

class Promo implements HasLevel {
    public $id;
    public $name;
    public $group;

    public function __construct($id, $name, $group)
    {
        $this->id = $id;
        $this->name = $name;
        $this->group = $group;
    }

    public function getLevel()
    {
        return $this->group['level'];
    }
}
class Tree {
    /**
     * @var Node
     */
    public $node;

    /**
     * @param HasLevel[] $objects
     * @return self
     */
    public function collectFromObjects($objects)
    {
        $levels = [];

        foreach ($objects as $object) {
            $levels[$object->getLevel()][] = $object;
        }

        ksort($levels);

        $this->node = $node = new Node();

        foreach ($levels as $level => $items) {
            $node->items = $items;
            $node->level = $level;
            if ($level === count($levels) - 1) {
                break;
            }
            $node->childNode = new Node();
            $node = $node->childNode;
        }

        return $this;
    }

    /**
     * @return ?Node
     */
    public function findByLevel($level)
    {
        if (!isset($this->node)) {
            return null;
        }

        $curNode = $this->node;

        while ($curNode->getLevel() !== $level) {
            $curNode = $curNode->childNode;

            if ($curNode === null) {
                break;
            }
        }

        return $curNode;
    }
}

$objects = [
    new Promo(3, 'Слава Украине', ['parent_id' => 2, 'level' => 2]),
    new Promo(1, 'Слава России', ['parent_id' => null, 'level' => 0]),
    new Promo(2, 'Слава Молдове', ['parent_id' => 1, 'level' => 1]),
    new Promo(12, 'Слава Беларуси', ['parent_id' => 2, 'level' =>2])
];


$tree = (new Tree())->collectFromObjects($objects);
$secondLevel = $tree->findByLevel(2);