实现代码：

<?php
//将验证码保存到session里，供全局使用
session_start();
$nums = "";
for($i=0;$i<4;$i++){
//产生随机数并转换成十六进制
$nums.=dechex(mt_rand(0,15));
}
//将验证码写入session
$_SESSION['code']=$nums;

//设置验证码长和宽
$_width = 60;
$_height = 20;
//创建一张图片
$_img = imagecreatetruecolor($_width,$_height);
//创建一个白色
$_white = imagecolorallocate($_img,220,250,250);
//填充背景
imagefill($_img,0,0,$_white);

//随机画6条线条
for($i=0;$i<6;$i++){
$_rnd_color = imagecolorallocate($_img,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
imageline($_img,mt_rand(0,$_width),mt_rand(0,$_width),mt_rand(0,$_width),mt_rand(0,$_width),$_rnd_color);
}

//随机画出雪花
for($i=0;$i<60;$i++){
imagestring($_img,1,mt_rand(1,$_width),mt_rand(1,$_height),"*",imagecolorallocate($_img,mt_rand(200,255),mt_rand(200,255),mt_rand(200,255)));
}

//输出验证码
for($i=0;$i<strlen($_SESSION['code']);$i++){
imagestring($_img,mt_rand(6,10),$i*$_width/4+mt_rand(1,10),mt_rand(1,$_height/2),$_SESSION['code'][$i],imagecolorallocate($_img,mt_rand(0,100),mt_rand(0,150),mt_rand(0,200)));
}

//输出和销毁
header("Content-Type:image/png");
imagepng($_img);
imagedestroy($_img);
?>