php - 密码保护的 php 页面的非常小的 php 代码，显示未定义的索引：

Answer 1

$USERS[$_POST["username"]]

when you leave the username input parameter blank, then it'll evaluate to:

$USERS[""]

what is a key that doesn't exist:

check for:

if(isset($_POST['ac'], $_POST["username"], $USERS[$_POST["username"]], $_POST["password"])) { /* ... */ }

instead of only checking for if sent:

if(isset($_POST['ac'])) { /* ... */ }

Answer 2

This error comes from the fact that you're using the user input $_POST['username'] as a key into the array $USERS without checking first that the key exists.

You could use isset to check that the key exists before accessing it, like this:

if (isset ($USERS[$_POST['username']]) && $USERS[$_POST['username']] == $_POST['password'])...