PHP if statement with isset not working

I presumed this was easy code but this doesnt work as intended:

$id = $_GET['id'];
if (!isset($id)){
    header("HTTP/1.1 404 not found");
    include("404.php");
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else {
    include('includes/functions.php'); 
}

So i've taken the get parameters off of the URL yet i dont receive a 404 error?

Any ideas?

You must do the test on $_GET['id'] directly:

if(!isset($_GET['id']) { ... }

Even if $_GET['id'] isn't set, since you tried to assign it to $id, $id starts to exist, so the test returns true and thus you don't have a 404 error.

You are checking if $id is set. And it is :

$id = $_GET['id'];

So you must check if the get variable is set:

isset($_GET['id'])

you have created $id variable in first line. You should check $_GET['id']. Also you can check this variable for empty.