What Are the Differences Between PSR-0 and PSR-4?

Recently I've read about namespaces and how they are beneficial. I'm currently creating a project in Laravel and trying to move from class map autoloading to namespacing. However, I can't seem to grasp what the actual difference is between PSR-0 and PSR-4. **Some resources that I've read are...** - [Battle of the Autoloaders][1] - [Laracasts PSR-4 autoloading][2] - [PSR-0][3] - [PSR-4][4] **What I understand:** - PSR-4 does not convert underscores to directory separators - Certain specific rules of composer cause the directory structure to become complex which in turn makes PSR-0 namespacing verbose and thus PSR-4 was created Examples explaining the difference would be appreciated. [1]: http://www.sitepoint.com/battle-autoloaders-psr-0-vs-psr-4/ [2]: https://laracasts.com/lessons/psr-4-autoloading [3]: http://www.php-fig.org/psr/psr-0/ [4]: http://www.php-fig.org/psr/psr-4/