Replacing a Built-in PHP Function When Testing a Component

Recently I needed to test part of Slim that uses the built-in PHP functions header() and headers_sent(). To do this, I took advantage of PHP’s namespace resolution rules where it will find a function within the same namespace first before finding one with the same name in the global namespace. The idea of how to do this came courtesy of Matthew Weier O’Phinney where this approach is used for similar testing in Zend-Diactoros.
This is the relevant part of the code I want to test: