<html>
<title>用户注册</title>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8">
<script language="javascript">
function check()
{
if(myform.username.value == "")
{
alert("用户名不能为空！");
myform.username.focus();
return false;
}
if(myform.pwd.value == "")
{
alert("密码不能为空！");
myform.pwd.focus();
return false;
}
if(myform.ccode.value == "")
{
alert("验证码不能为空！");
myform.ccode.focus();
return false;
}
return true;
}
</script>
</head>
<body>
<form action="" method="post" onsubmit="javascript:return check()" name="myform">
<label for="user">姓 名：</label>
<input type="text" name="username" id="user"/><br>
<label for="pwd">密 码：</label>
<input type="password" name="pwd" id="pwd"/><br>
<label for="ccode">验证码：</label>
<input type="text" name="ccode" id="ccode">
<?php
$num=mt_rand(1000,9999);
for($i=0;$i<4;$i++)
{
$n=substr($num,$i,1);
echo "<image src='images/code/$n.gif'>";
}
?>
<br>
<input type="submit" name="sub" value="提交"/>
</form>
</body>
</html>原文：http://blog.csdn.net/cjc211322/article/details/45116419