利用session生成唯一自增4位数数字

<?php

define("TODAY_DATE", date('y') . date('m') . date('d'));

class StringTools
{
    private static $StringTools;
    private  function __construct(){}
    private  function __clone(){}

    public static function  getStringTools(){
        if(!self::$StringTools instanceof self){
            self::$StringTools = new self();
        }
        return self::$StringTools;
    }

    public static function makeCode()
    {
        if (isset($_SESSION['mt_rand_num']) && TODAY_DATE == $_SESSION['mt_rand_num']['date']) {
            $_SESSION['mt_rand_num']['num'] = intval($_SESSION['mt_rand_num']['num'] + 1);
            $res = TODAY_DATE . sprintf("%04d", $_SESSION['mt_rand_num']['num']);
        } else {
            $mt_rand_num['date'] = TODAY_DATE;
            $mt_rand_num['num'] = 1;
            $_SESSION['mt_rand_num'] = $mt_rand_num;
            $res = TODAY_DATE . sprintf("%04d", $mt_rand_num['num']);
        }
        return $res;
    }




}