Can't set empty variable even if the statement is correct?

I have a website where you visit where the URL parameters define where you should go, but now I also want a default state, where you get send when you visit without any URL parameters.

This is what it looks like:

$link = "http://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";


ob_start(); 
echo strstr($link, 'directory');
$link2 = ob_get_contents();
ob_end_clean();

echo("$link2"); //the reason I have this was to test, it returns empty

if (!empty($_GET['link2'])) {
$link2 = "directory=four";
}

So what it does is just get the URL, remove everything that comes before directory, but this doesn't set the directory to four, but here it just goes to localhost/guards/game.php?

$aaaa=("http://localhost/guard/game.php?$link2");
?>

<script type="text/javascript">
window.location = "<?php echo $aaaa ?>";
</script>