Create a tree structure from a flat structure efficiently using variable addresses

category_tree.php

protected function getCategoryTree()
    {
        // IMPORTANT: Return categories sorted by parent_id ASC
        $categories = $this->em->getRepository('ZumnyCoreBundle:Category')->findAll();

        $flat = array();
        $tree = array();

        /** @var EntityCategory $category */
        foreach ($categories as $category) {
            $category = $category->toArray();
            $category['children'] = array();

            if (!isset($flat[$category['id']])) {
                $flat[$category['id']] = $category;
            }

            if (null == $category['parent']) {
                $tree[] = &$flat[$category['id']];
            } else {
                $flat[$category['parent']]['children'][$category['id']] = &$flat[$category['id']];
            }
        }

        return $tree;
    }