php如何实现验证码看不清换一张的效果

<?php
 
session_start();
function random($len) {
 $srcstr = "1a2s3d4f5g6hj8k9qwertyupzxcvbnm";
 mt_srand();
 $strs = "";
 for ($i = 0; $i < $len; $i++) {
 $strs .= $srcstr[mt_rand(0, 30)];
 }
 return $strs;
}
 
//随机生成的字符串
$str = random(4);
 
//验证码图片的宽度
$width = 50; 
 
//验证码图片的高度
$height = 25; 
 
//声明需要创建的图层的图片格式
@ header("Content-Type:image/png");
 
//创建一个图层
$im = imagecreate($width, $height);
 
//背景色
$back = imagecolorallocate($im, 0xFF, 0xFF, 0xFF);
 
//模糊点颜色
$pix = imagecolorallocate($im, 187, 230, 247);
 
//字体色
$font = imagecolorallocate($im, 41, 163, 238);
 
//绘模糊作用的点
mt_srand();
for ($i = 0; $i < 1000; $i++) {
 imagesetpixel($im, mt_rand(0, $width), mt_rand(0, $height), $pix);
}
 
//输出字符
imagestring($im, 5, 7, 5, $str, $font);
 
//输出矩形
imagerectangle($im, 0, 0, $width -1, $height -1, $font);
 
//输出图片
imagepng($im);
 
imagedestroy($im);
 
$str = md5($str);
 
//选择 cookie
//SetCookie("verification", $str, time() + 7200, "/");
 
//选择 Session
$_SESSION["verification"] = $str;
?>

<img id="checkpic" onclick="changing();" src='/images/checkcode.php' />

function changing(){
 document.getElementById('checkpic').src="/images/checkcode.php?"+Math.random();
}