To this date, the PHP-FIG group has released two autoloading standards. Predating PSR-4 was PSR-0. It was the first standard released by the PHP-FIG group. Its class naming had certain backward compatibility features aligned with an even older PEAR standard. Whereas, each level of the hierarchy was separated with a single underscore, indicating pseudo-namespaces and directory structure. The PHP 5.3 release then brought official namespace support to the language. PSR-0 allowed both the old PEAR underscore mode and the use of the new namespace notation. Allowing the underscores for some time to follow eased the transition to namespaces and encouraged wider adoption. Pretty soon, Composer came on the scene.
Note
Composer is a popular dependency manager for PHP that deals with packages and libraries by installing them in a vendor/ directory of our project.
With Composer's vendor/ directory philosophy, there was no single main directory for PHP sources as with PEAR....
