The SitePoint PHP blog has a new post from editor Bruno Skvorc about a PSR standard (from the PHP-FIG group) that's proposing and update and slight change to the currently wide-practiced autoloading standard (PSR-0). This new standard, PSR-4, proposes a modification to the PSR-0 spec with a bit more strict guidelines.

If you've gone past the beginner stage in your PHP training, you've heard of PSR-0 – an autoloading standard that defines ways to automatically include PHP classes in your code without having to use statements like require and include. [...] When Composer showed up and took the PHP package management world by storm, things changed. Due to some of its rules, folders often duplicated and became too deep when looking at PSR-0 class installations via Composer. [...] Therefore, some highly qualified PHP devs got together and put together a suggestion for a new standard: PSR-4.

The goal behind PSR-4 is to define a new autoloading standard that removes allowances for things like the underscaore as a "namespacing" tool. Bruno makes some suggestions for the structure of your tools if you're going to go with PSR-4 and the handling of multiple autoloaders/paths in the same namespace.