PHP 字符串切分拼接

<?php
$cache_path = "/part/to/cache";
$new_group_id = $group_id;
$len = strlen($group_id);
if($len < 4){
    $new_group_id= sprintf("%04d", $group_id);
}
$dir_one = substr($new_group_id, 0, 2);
$dir_two = substr($new_group_id, 2, 2);
$file_name =  $cache_path.'/'.$dir_one.'/'.$dir_two.'/'.$group_id.'.json';