php如何写注册登录验证码

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>&nbsp;</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>
<?
 
 
 
} 
}
?>

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