you would put a script like this
<?php
if (!isset($display)) {
$display = "home";
}
if ($display == "home") {
//do something
} elseif ($display == "about") {
//do something
} else {
//do something
}
?>
of if you wanted to use includes
<?php
if (!isset($display)) {
$display = "home";
}
if (file_exists($display.".html")) {
include($display.".html");
} else {
include("home.html");
}
?>