Simple PSR4 Autoloader

autoloader.php

<?php

spl_autoload_register(function($class) {
        // Replace with your root name space
        $root = "Foo\\";
  
        // Remove the root namespace
        if (substr($class, 0, strlen($root)) == $root) {
            $relative = substr($class, strlen($root));
        } 

        // Replace with your root directory
        $base_dir = __DIR__ . "/classes/";
    
        $filename = $base_dir . str_replace('\\', '/', $relative) . ".php";

        // Check if the file exists
        if (file_exists($filename)) {
            require_once($filename);
            if (class_exists($class)) {
                return true;
            }
        }

        return false;
});