recursive func for dropdown (php)

gistfile1.phtml

    $folders = $this->files_model->select('id,file_parent_id,file_display_name,file_type')->where(array('file_type' => 'folder'))->find_all();

		$indent = array();
		$get_menu = function($folders, $folder_id = 0) use(&$get_menu, &$indent) {

			$menu_html = '';
			foreach($folders as $folder)
			{
				if($folder->file_parent_id == $folder_id) {
					if($folder_id > 0)
						$indent[$folder->id] = $indent[$folder->file_parent_id]+1;
					else
						$indent[$folder->id] = 0;

					$menu_html .= '<option value="'.$folder->id.'">';
					for($i = 0; $i < $indent[$folder->id]; $i++)
					{
						$menu_html .= '---';
					}
					$menu_html .= $folder->file_display_name.'</option>';
					$menu_html .= $get_menu($folders, $folder->id);
				}
			}
			return $menu_html;
		};

		echo '<select>'.$get_menu($folders).'</select>';