Description:
------------
Just about the same bug I've reported in #47593
This time ReflectionFunction fails to work with functions with fully qualified namespaces.

Reproduce code:
---------------
namespace ns;
function func(){}
//new \ReflectionFunction('ns\func'); //Works
new \ReflectionFunction('\ns\func'); //Throws exception

Expected result:
----------------
new ReflectionFunction object created

Actual result:
--------------
Fatal error: Uncaught exception 'ReflectionException' with message 'Function \ns\func() does not exist' in ...