What does mean if (\false) (yes, with backslash) in PHP?

This is a no sense code, simply this is a Unreachable code because \false is always false!

It means it's using the false defined in the global namespace..

After a bit of research it turns out the rest of this answer is nonesense... I could swear you were able to do this in PHP at one point in time.

I think this is get around the situation where

<?php
namespace whywouldyoudothis;

false = true;
?>

I have never ever seen anyone code for this but that's what springs to mind.

if (\false) {
    class TextExtension extends \Twig_Extensions_Extension_Text
    {
    }
}

The code is still reachable by code sniffers and IDEs. However I think it should have deprecation note there. So that developers would be notified about using deprecated classes.

Here is an example from main Twig repository. https://github.com/twigphp/Twig/blob/v2.10.0/lib/Twig/Token.php

From php manual

Prefixing a name with \ will specify that the name is required from the global space even in the context of the namespace.