php 新手登陆问题

<?php
/*
* Created on 2011-9-4
*
* To change the template for this generated file go to
* Window - Preferences - PHPeclipse - PHP - Code Templates
*/
include("head.php");
include("conn.php");
if($_POST[id]=='admin'){
        $pw=md5($_POST[pw]);
                if($pw=='21232f297a57a5a743894a0e4a801fc3'){
                        setcookie("login","ok");
                                echo        "<script language=\"javascript\">location.href='login.php';</script>";
        }
}
if($_cookie['cookie']!='ok'){
?>
<script language=javascript>
function checklogin(){
        if(login.id.value==""){
                alert("用户名不能为空");
                login.id.focus();
                return false;
        }
        if(login.pw.value.length<5){
                alert("密码不能少于5个字符");
                login.pw.focus();
                return false;
        }
}

</script>
<form action="" method="post" name="login" onsubmit="checklogin()">
        用户名：<input type="text" name="id" /><br />
        密码: <input type="password" name="pw" /><br />
        <input type="submit" name="submit" value="登录" >
<?php
}else{
?>
<a href="add.php">退出</a>
<?php
}
?>
</form>
这段代码哪错了，提示错误
Notice: Use of undefined constant id - assumed 'id' in D:\WampServer-x64\wamp\www\leaveword\login.php on line 10
Notice: Undefined index: id in D:\WampServer-x64\wamp\www\leaveword\login.php on line 10
Notice: Undefined variable: _cookie in D:\WampServer-x64\wamp\www\leaveword\login.php on line 17

$_POST[id]改成$_POST['id']