Difference in loading via the spl_autoload_register

Question

I have three files

/autoload-test.php

spl_autoload_register(function ($class) {
    require_once($_SERVER['DOCUMENT_ROOT'] . '/controllers/' . $class . '.php');
});
$different_file_obj = new test;

/controllers/test.php

require_once('some_var.php');
class test {
        function __construct() {
                echo $GLOBALS['var'];
            }
    }

/controllers/some_var.php

$var = 'some value';

When I open the file 1 in browser it gives me this -> Notice: Undefined index: var in /Applications/XAMPP/xamppfiles/htdocs/avrs/controllers/test.php on line 7

But if I change the file to ->

require_once($_SERVER['DOCUMENT_ROOT'] . '/controllers/test.php');
$different_file_obj = new test;

It works fine, so I am thinking that there is some difference in including the file using the spl_autoload_register, because that is the only thing that I am changing here. Including with out the spl_autoload_register works absolutely fine.

Is there something I should know or some where I can read in details about this?