Fallback Function

A fallback function is a function that replaces a function if its definition is not available in the current namespace.

PHP has implemented a mechanism so that functions that are undefined in the current namespace, have a fallback in the global space. This is important for backward compatibility, and the native PHP versions.

<?php

namespace A {
     // A\strtolower doesn't exist,
     // it falls back on \strtolower(), in the global space
     echo strtolower('A');
}

?>

Documentation