Reports the use statements that contain non-fully qualified import names and thus have no effect.

Import names must be fully qualified. Note that for namespaced names (that is, FQNs containing the namespace separator, such as Foo\Bar), using the leading backslash is not necessary and not recommended. For global names that do not contain the namespace separator, such as FooBar, using the leading backslash is required.

See Using namespaces: Aliasing/Importing (php.net) for details.