RainLab\Blog\Models\Category::listSubCategoryOptions PHP Method

    protected static function listSubCategoryOptions()
    {
        $category = self::getNested();
        $iterator = function ($categories) use(&$iterator) {
            $result = [];
            foreach ($categories as $category) {
                if (!$category->children) {
                    $result[$category->id] = $category->name;
                } else {
                    $result[$category->id] = ['title' => $category->name, 'items' => $iterator($category->children)];
                }
            }
            return $result;
        };
        return $iterator($category);
    }