Drupal 8 embraces modern PHP with all the trimmings, shedding the baggage of supporting earlier PHP versions and embracing the new object-oriented features.

One such feature is namespaced objects and the PSR-0 standard for autoloader compatability.

But if you know your project will run on a recent version of PHP, there's no reason you can't write your custom modules using PSR-0 now, in Drupal 7