PHP生成32位随机字符串

 public function get_str(){
        
          $length = 32;
          $str = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
             $len = strlen($str)-1;
             $randstr = '';
             for ($i=0;$i<$length;$i++) {
              $num=mt_rand(0,$len);
              $randstr .= $str[$num];
             }
             return $randstr;
        
    }