spl_autoload_register does not work

Question

I am developing a system in mvc. Developing locally, everything worked correctly... But when I put it online to be able to test, it just didn’t load, it was 500. So I started trying to find the error. I put echo 'test' inside spl_autoload_register to see if it was entering the function, but it is not. It does not find my core class because it does not enter the spl_autoload_register function :(

I can’t imagine what it might be, if it’s some server setup or something. I’m locally running php 7.4 and online 7.2

Mistake was:

PHP Fatal error: Uncaught Error: Class 'Core' not found in /home/server/public_html/index.php:31 Stack trace: #0 {main} thrown in /home/server/public_html/index.php on line 31

<?php

    session_start();

    // Connection to database
    require('config.php');

    // Checking if the files exist in 'controller', 'core' and 'models'
    spl_autoload_register(function($class) {

        if(file_exists('sources/controllers/'.$class.'.php')) {

            // Controllers
            require 'sources/controllers/'.$class.'.php';

        } else if(file_exists('sources/models/'.$class.'.php')) {

            // Models
            require 'sources/models/'.$class.'.php';

        } else if(file_exists('core/'.$class.'.php')) {

            // Core
            require 'core/'.$class.'.php';

        }

    });

    // Class core
    $core = new Core();

    $core->start();

?>