虽然代码工作正常，但未定义的索引错误？ [重复]

My code is working fine but every time the page has loaded an error shows at the top of the screen saying:

Undefined index: userKeySubmitted in C:\xampp\htdocs\Lunabooster 2\cp\index.php on line 4

Note that the login system

I've tried rearranging the code and what not but still same error.

Here's my full PHP code:

<?php
session_start();

$userKey = $_POST['userKeySubmitted'];
$keylist = file ('priv/keys.txt');

$success = false;
foreach ($keylist as $key) {
    $user_details = explode('|', $key);
    if ($user_details[0] == $userKey) {
        $success = true;
        if ($success) {
            $_SESSION['userKey'] = $userKey;
        header('Location: user.php');
        break;
    } else {
        break;
    }
    }
}

?>

</div>

Because when the page load, there is no userKeySubmitted index You need to check if the index exist first :

$userKey = isset($_POST['userKeySubmitted']) ? $_POST['userKeySubmitted'] : null;