The standard library isn't namespaced. If you want to use a stdlib function inside an NS, you can without issue. If you want to use a stdlib class, or any top level class, it needs backslash before its name or you need to import it.

There is a community move towards a standard namespacing with PHP-FIG. This is useful and we are seeing lots of progress on internals thanks to the work by the community.

Like a lot of things, PHP name spaces are or were a big mess. Lots of progress is being made to improve though I think a lot must remain.

I've thought about creating a project that packages up various categories in the stdlib into namespaces with consistent inputs and outputs. That would be nice but the process isn't a lot of fun.