Trying to make chat room for my browser game

This is my first post, I'm trying to make chat-room for my browser game and I want to make when user register on my browser game to be able when go to chat room to use that username. This is error "Notice: Undefined index: id in *:*****php." and I keep hitting the wall this is my code:

<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>

This is picture of error:

it seems messages to work at least

P.S I'm new to programming and I was looking for this problem over the internet, can't find it. So I ask you smart people to help me pls :)

Solution

Like @Suneel Kumar commented on your question. Your error lies here:

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

You are selecting the following rows from your database: messageid, message, timestamp

And you are trying to echo an undefined index of an array "id".

echo $row['id']

Either you have to change this to

echo $row['messageid']

or you have to query the id row from the table aswell.

Edit:

A good "debugging" tip for developing in PHP would also be to try something like var_dump($row). This would dump the entire array object in your browser, so that you can see all the key values that exists in the object you are working with.