<?php
require "session.php";
if(!$user){
    header("Location: login.php?dest=".urlencode($_SERVER["REQUEST_URI"]));
    die();
}
?>

<?php
require "scripts/session.php";
$dest = "./";
if(isset($_GET["dest"])){
    $dest = $_GET["dest"];
}
?>

var URL = "<?php echo $dest; ?>";
//...
//upon successful login (via AJAX):
window.location.replace(URL);