Check if function belongs to namespace or global

I need check if called function belongs to the current namespace or it is defined globally.

<?php

namespace Test;

function is_string($str) { return false; }

var_dump(is_string('test'));  // will be always false, is the namespaced is_string()
var_dump(\is_string('test')); // will be the global is_string()

Now I need check if is_string() or \is_string() is defined inside the namespace or is global, and it will occur at the same file. Sometimes the namespaced version will not be defined.

I have tried to use the ReflectionFunction class, but it no works:

var_dump((new \ReflectionFunction('is_string'))->inNamespace());      // returns false, instead of true
var_dump((new \ReflectionFunction('is_string'))->getNamespaceName()); // returns '', instead of 'Test'

var_dump((new \ReflectionFunction('\\is_string'))->inNamespace());      // returns true, that is right
var_dump((new \ReflectionFunction('\\is_string'))->getNamespaceName()); // returns '', that is right

Seems that it always points to the global, but when I just run is_string() it uses the namespaced version (that is correct on this context).