<?php
$pages = ["intro", "print-server", "print-get", "server-lengths"];
$dir  = __DIR__ . "/content";
$file = null;
$page = isset($_GET["subpage"]) ? $_GET["subpage"] : "intro";

if (in_array($page, $pages, true)) {
    $file = "$page.php";
} else {
    $file = "404.php";
}
?>
<aside>
    <?php include("multi/aside.php");?>
</aside>
<article>
    <?php include("content/$file")?>
    <?php include("incl/byline.php"); ?>
</article>
