覆盖特定类的PHP命名空间

If I have a PHP namespace as follows:

<?php

   namespace A\B;

   $test = new MyClass(); 

it seems every time I create a new instance of class, the name is prefixed with the namespace, e.g.

$test = new A\B\MyClass();

What happens if I don't want to use a namespace for another class, e.g. I want to call

$test = C\D\AnotherClass();

Currently this becomes:

$test = A\B\C\D\AnotherClass();

which results in an error.