PHP Notice: Undefined index: action in C:\wamp\www\cookie\login.class.php on

<?php
require 'config.inc.php';
function clearCookies() {
setCookie('username','',time()-3600);
setCookie('isLogin','',time()-3600);
}
if($_GET["action"]=="login"){
clearCookies();
$usercheck='select * from members where username="'.$_POST['username'].'"'; //将sql的查询语句赋值到变量$usercheck,{$_POST["username"]}是通过POST方法获取表单的值。
$back=mysql_query($usercheck); //mysql_query() 函数执行一条 MySQL 查询
$rows=mysql_fetch_row($back);
if($_POST['username']==$rows[1]&&$_POST['password']==$rows[2]) {
setCookie('username',$_POST["username"],time()+60*60*24*7);
setCookie('isLogin','1',time()+60*60*24*7);
header("Location:index.html.php");
}else{
die("用户名或密码错误!");
}
}else if($_GET["action"]=="logout"){
clearCookies();
}
?>
怎么修改

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