Even when autoloading (SPL) is used, class inheritance does not seem to work. Simply the PHP engine is unable to find parent (inherited) class. PHP 5.6 and 7.0 behave exactly same on this, which beats the purpose of autoloading.
And IMHO it's easy to fix as the autoloader is able to find all first level classes w/o problems, it just needs to follow same path recursively on parents too.
<?php
spl_autoload_register();
spl_autoload_register(function ($class){
require_once __DIR__ . '/' . strtolower(str_replace('\\', '/', $class) . '.php');
});