Because PHP basically fail to distinct between for instance foo::bar(); as in namespace foo with function bar vs. class foo with static function bar they’ve just decided to scrap the whole :: thing and instead use backslash. I can’t tell you how stupid I think this really is. Instead of actually fixing the problem, faulty look-up, they’ve taken the very easy way out. This just adds to the inconsistencies for PHP. There are several programing languages that I know of that don’t have these problems, and none of them have namespace separation in this way. Actually, I can’t even remember ever seeing any language using backslash for anything other than escaping, but that might just have something to do with other languages being consistent. This quote from a comment on slashdot really says it all:

Java:
Attribute/Method access: foo.bar
Static method access: Foo.bar
Package access: foo.bar.baz

C#:
Attribute/Method access: foo.bar
Static method access: Foo.bar
Namespace access: foo.bar.baz

Python:
Attribute/Method access: foo.bar
Static method access: Foo.bar
Module access: foo.bar.baz

PHP:
Attribute/Method access: $foo->bar
Static method access: Foo::bar
Namespace access: foo\bar\baz

Now I’m not quite done here. If you look at the RFC at PHP about the change they list “IDE compatibility” as criterion for the new namespace separator. This is just plain stupid. Who are you designing a language for, humans or IDEs? I would say it’s for humans, but it seems the PHP devs thinks otherwise …