I'm trying to figure out why would underscores in class names be converted to directory separators in PSR-0? I'm sure there is a reason for it, but I can't think of why.
If I have a class named my_class_name in the namespace amespace\subnamespace Wouldn't that then convert to amespace\subnamespace\my\class ame when what I want is amespace\subnamespace\my_class_name? I understand if my class system is built around this it would work, but if I wanted directory separators I would have just used a backslash instead of an underscore.
Because history. It was common prior to PHP gaining namespace support for projects to use underscore-separated bits for a sort of poor man's namespacing. There was enough of that still going on at the time PSR-0 was created to make it a practical decision.