The specification currently states that use declarations bring a name into a scope, but that's not exactly true, as imports are not available prior to the use declaration.

namespace A { class C {} } namespace B { // Same scope, 'A\C' should be available right?! $c = new C(); use A\C; }

Without this restriction, an engine could theoretically make an initial pass to import names from all use declarations, and therefore prevent a name resolution error from occurring.

18-namespaces.md

A namespace-use-declaration imports — that is, makes available — one or more names into a scope, optionally giving them each an alias.