Current Guidelines have explaination for PSR-0

{{{
A PSR-0 [1] compliant library would put its PHP files in /usr/share/php/<Vendor Name>
}}}

PSR-0 use a full tree which match class namespace, which is perfect for downstream.

But a lot of library start to switch to PSR-4. In this new standard, some top directories of the PSR-0 tree can be dropped from the sources tree. (A PSR-4 autoloader have to be aware of this dropped prefix).

PSR-4 have no sense for downstream distribution.

Proposal: restore a PSR-0 compliant tree to avoid any name conflict. Such an installation can be used from a PSR-0 or PSR-4 autoloader:

• PSR-0 autoloader config: Foo\Bar are in /usr/share/php
• PSR-4 autoloader config: Foo\Bar are in /usr/share/php/Foo/Bar

In both case Foo\Bar\Pop class is /usr/share/php/Foo/Bar/Pop.php

Proposed changes to PHP Guidelines

Just add

{{{
A PSR-4 [2] compliant library would put its PHP files
in /usr/share/php/<Vendor Name> in a PSR-0 compliant tree.
}}}

With link to https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-4-autoloader.md

This change is mostly only to avoid ambiguity, some PHP packaged libraries already follow this (sometime, simply because upstream have switch from PSR-0 to PSR-4 in their sources repository)