PLEASE NOTE: I inverted the actual and expected results, sorry.

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

This is expected, see #51651

But then PHP's behavior is incoherent because:

<?php
namespace NS;

class HelloWorld {}
function hello_world() {}

// This will work
new HelloWorld;

// This will fail
hello_word();
?>

And anyway, being able to call bar() while 'bar' isn't 
considered callable by is_callable() (see report) is totally 
incoherent. Or I'm missing something big.

It is not incoherent, see...

"How does an unqualified function name or unqualified constant name like name resolve?

Function or constant names that do not contain a backslash like name can be resolved in 2 different ways. 

First, the current namespace name is prepended to name. 

Finally, if the constant or function name does not exist in the current namespace, a global constant or function name is used if it exists."

-- http://docs.php.net/manual/en/language.namespaces.faq.php

namespace foo;
function bar() { }

bar(); // foo\ is prepended in compile-time

$x = 'bar'; // obviously foo\ will not be prepended
$x(); // hence 'bar' is not found

Great, I'm okay with that.

But how do explain this?

<?php
namespace Foo;

function bar() {}
function hello_world() {}

bar(); // works
hello_word(); // fails

You mistyped 'world' on function call :-)

Damn it, thanks!
Well, sorry to have bothered you and thank you for answering my 
questions. ;)