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);

?>