Before PHP 5.3, due to the lack of a namespace construction, PEAR-style namespaces were used. PEAR is an acronym for PHP Extension and Application Repository, and in the good old days, it was a Repository of reusable components. It's still active, but it's not very convenient, and not many people use it anymore — particularly since Composer and Packagist were introduced. PEAR, as a source of reusable components, needed a way to avoid class name collisions, so contributors started prefixing class names with namespaces. There are still projects that use this form of namespaces (PHPUnit and Zend Framework 1, to name a couple). An example of PEAR-style namespaces:

The following would be an example of PEAR-style namespaces:

                 

The class name for the Bill entity, using PEAR-style namespaces, would become BuyIt_Billing_Domain_Model_Bill_Bill. However, this is a bit ugly, and it doesn't follow one of the main Domain-Driven Design mantras: every class name should be named in terms of the Ubiquitous Language. For this reason, we strongly discourage its use.