PHP验证码常用方法（比较简单）

文件一:authform.php
<script language="javascript">
function check(){
if(document.forms[0].num.value==''){
alert("请输入验证码");
return false;
exit;
}
return true;
}
</script>
<form action=authpage.php method=post onsubmit="return check();">
<table>
请输入验证码：<input type=text name=num style="width:80px"><br>
<input type=submit name="验证" value="提交验证码">
<img src=authimg.php>
</table>
</form>
文件二：authimg.php
<?php
Header("Content-type:image/PNG");
session_start();
$auth_num="";
session_register('auth_num');
$im=imagecreate(63,20);
srand((double)microtime()*1000000);
$auth_num_k=md5(rand(0,9999));
$auth_num=substr($auth_num_k,17,5);
$black=ImageColorAllocate($im,0,0,0);
$white=ImageColorAllocate($im,255,255,255);