php注册登录时生成的验证码

http://blog.sina.com.cn/s/blog_8173443e01012l82.html

记得我学php时第一件事就是研究登陆注册。当然，登陆少不了验证码。两年过去了，昨天突然想用个验证码却找不到代码了，现在找到了，贴出来吧。防止我这个马大哈再找不到。随机生成四位数字符串，可以是数字也可以是数字和字母。

authimg.php生成验证码的文件：
<?

$IsLetter = false; //是否出现字母
session_start();
$Num = $_GET["n"];

//生成验证码图片
header("Content-type: image/PNG");
$im = imagecreate(46,20);
srand((double)microtime()*1000000);
$Red = rand(0,200);
$Green = rand(0,200);
$Blue = rand(0,200);
$Color = imagecolorallocate($im, $Red, $Green, $Blue);
$BackGround = imagecolorallocate($im, 255,255,255);
imagefill($im,0,0,$BackGround);

if($IsLetter)
{
$a = substr(md5($Num*10000000000000000),0,1);
$b = substr(md5($Num*10000000000000000),4,1);
$c = substr(md5($Num*10000000000000000),8,1);
$d = substr(md5($Num*10000000000000000),12,1);
}
else
{
$a = substr(hexdec(md5($Num*10000000000000000)),2,1);
$b = substr(hexdec(md5($Num*10000000000000000)),3,1);
$c = substr(hexdec(md5($Num*10000000000000000)),4,1);
$d = substr(hexdec(md5($Num*10000000000000000)),5,1);
}

$Authnum = strtoupper($a.$b.$c.$d);

$_SESSION["Authnum"] = $Authnum;

imagestring($im, 5, 5, 2, $Authnum, $Color);
for($i=0;$i<200;$i++) //加入干扰象素
{
$randcolor = imagecolorallocate($im,rand(0,255),rand(0,255),rand(0,255));
imagesetpixel($im, rand()p , rand()0 , $randcolor);
}
imagepng($im);
imagedestroy($im);
?>

需要显示验证码的文件：

<?
session_start();

$AuthInput = $_POST["AuthInput"];
$action = $_GET["action"];
?>
<script language="javascript">
function lyalert(){
if( addform.AuthInput.value=="") {
alert("请填写验证码");
document.addform.AuthInput.focus();
return false;
}
}
</script>
<form name="addform" method="post" action="?action=save" id="addform" onsubmit="return lyalert()">
<table width="500" border="1" cellpadding="5" cellspacing="0">
<tr><td>验证码：</td>
<td><input name="AuthInput" type="text" id="AuthInput" size="8" maxlength="4" /><script>document.write("<img src=\"authimg.php?n=",Math.random(),"\" />");</script></td></tr>
<tr><td colspan="2" align="center"><input type="submit" name="Button1" value="确定" id="Button1" /></td>
</tr>
</table>
<p> </p>
</form>

<?
if($action=='save')
{
if ($_SESSION["Authnum"] != $AuthInput)
{
?>
<script language="javascript">
alert("验证码输入不正确");
</script>
<?
}
else
{
?>
<script language="javascript">
alert("提交成功！");
</script>
<?

}
}
?>