正在尝试为我的浏览器游戏创建聊天室

这是我的第一篇帖子,我正在尝试为我的浏览器游戏创建聊天室,我想让用户在我的浏览器游戏上注册时能够在聊天室使用该用户名。这是错误“注意:未定义的索引:id in*:****php”。我不断碰壁,这是我的代码:

<html>
<head>
    <meta http-equiv="refresh" content="2">
</head>
<body>
<?php
    //Database connection
    $*********=             "******";
    $**********=                "*****";
    $**********=                "*******";
    $*****=                 "*********";

    //Create Connection
    $conn = new mysqli($********, $*******, $*******, $****);

    //Check Connection
    if($conn->connect_error)
    {
        die("Connection error: ".conn>connect_error);
    }

    if(isset($_SESSION['loggedin']))
    {
        $username = $_SESSION['loggedin'];

        //GET USER DATA
        $query = "SELECT id FROM users WHERE username = '$username'";
        $result = mysqli_query($conn, $query);
        $row = mysqli_fetch_assoc($result);

        //USERDATA
        $userId   =  $row['id'];
    }

    //query the database
    $query = "SELECT messageid, message, timestamp FROM messages ORDER by
    timestamp DESC";

    if ($result = mysqli_query($conn, $query))
    {
        /* fetch associative array */
        while ($row = mysqli_fetch_assoc($result))
        {
            echo $row['id'].' says: '.$row['message'].'</br>';
        }
        mysqli_free_result($result);
    }

    /* close connection */
    mysqli_close($conn);
?>
</body>
</html>

it seems messages to work at least

P、 我是编程新手,我在网上找这个问题,找不到。所以我请你们这些聪明人帮我:)