won't find the class


With the simple example i'm getting this: 
Fatal error: Uncaught Error: Class 'BackgroundWorkerManager' not found in /var/www/html/test/multitasking1/t.php:3
Stack trace:
#0 {main}
  thrown in /var/www/html/test/multitasking1/t.php on line 3

first 3 lines:
<?php
require __DIR__ . '/vendor/autoload.php';
$man = new BackgroundWorkerManager(2);