why the php condition not work unless i echo the variable?

in the code below, if I echo the variable the condition will work if I don’t echo the variable the condition will not work!

what is the error?

the code:

$msg=$_SESSION['$msg'];
echo $msg;
if($msg != null){ ?>
<script src="https://cdn.jsdelivr.net/npm/sweetalert2@9"></script>
<script >
    swal.fire({
                icon: "success",
                title: "success",
                showConfirmButton: false,
                timer: 1300
        })
 </script>

<?php } ?>

edited code: even this does not work unless echo!

$msg="ss";
if(!empty($msg)){ ?>

<script src="https://cdn.jsdelivr.net/npm/sweetalert2@9"></script>
<script >
    swal.fire({
                icon: "success",
                title: "success",
                showConfirmButton: false,
                timer: 1300
        })
 </script>

<?php } ?>

>Solution :

There are few problems in the code:

1. Call session_start(); before using $_SESSION.

2. $_SESSION['$msg'] can be undefined and can trigger notice. You should check if the key exists with isset($_SESSION['$msg']).

3. Session key $msg is a bit strange. You don’t need the $ for session keys.

4. If you want to check for not null, use strict comparison !==.

<?php 
session_start();
$msg = isset($_SESSION['$msg']) ? $_SESSION['$msg'] : null;
echo $msg;
if($msg !== null){ ?>
<script src="https://cdn.jsdelivr.net/npm/sweetalert2@9"></script>
<script >
    swal.fire({
                icon: "success",
                title: "success",
                showConfirmButton: false,
                timer: 1300
        })
 </script>

<?php } ?>