[php] Is it possible to overwrite a function in PHP

A solution for the related case where you have an include file A that you can edit and want to override some of its functions in an include file B (or the main file):

Main File:

<?php
$Override=true; // An argument used in A.php
include ("A.php");
include ("B.php");
F1();
?>

Include File A:

<?php
if (!@$Override) {
   function F1 () {echo "This is F1() in A";}
}
?>

Include File B:

<?php
   function F1 () {echo "This is F1() in B";}
?>

Browsing to the main file displays "This is F1() in B".