php四位数验证码，用到函数range,implode等

<?php
header('content-type:text/html;charset=utf-8');
$glue=range(chr(33), chr(126),1);
natsort($glue);
$string=implode($glue, '');
$glue=array();
for($i=0;$i<4;$i++){
    $glue[$i]=$string[mt_rand(0, strlen($string)-1)];
}
$string=implode($glue, '');
echo $string;