<?php
$page = isset($_GET["subpage"]) ? $_GET["subpage"] : "intro";
$multipage = [
    "intro"    => "intro.php",
    "current"  => "current-value.php",
    "get"      => "change-get.php",
    "dropdown" => "change-dropdown.php",
    "post-dropdown" => "post-dropdown.php",
    "postform" => "postform.php",
];

if (isset($multipage[$page])) {
    $file = $multipage[$page];
} else {
    $file = "404.php";
}
?>
<aside><?php include("stylechooser/aside.php"); ?></aside>
<article>
    <?php include("stylechooser/content/$file"); ?>
</article>
