<?php if($id == NULL) { include('home.php'); } else { include(''.$id.'.php'); } ?>
let's break the code a little:
if($id == NULL)If the $ID has no ?id=test or something it will include the home.php file
include('home.php');
else it will include the ?id=download and the file will be download.php
include(''.$id.'.php');