This is enough to run the site and test PDF export:

sudo apt update
sudo apt install php-cli php-xml php-mbstring php-gd

• php-cli: PHP itself and the built-in development server
• php-xml: the dom extension that Dompdf requires (a missing dom extension caused the earlier error)
• php-mbstring: Dompdf needs it for character set handling, including accented characters
• php-gd: only needed for image handling in PDFs, so you can skip it if your pages have no images

Verify

php -v
php -m | grep -i -E "dom|mbstring|gd"

Running the site

Run these in the project folder:

cd ~/Webdesign/testbed/CheapIndex
php -S localhost:8000

Open http://localhost:8000 in your browser. Changes to index.php and index.md show up as soon as you refresh the page. You only need to restart the server if you install or change PHP extensions.

A note on routes

The built-in server does not automatically route URLs like /contact or /about to index.php, so they return a 404 error. If you want to test the $url_routes feature locally, create a file called router.php in the project folder:

<?php
// Let existing files through (css, images, helpers/pdf.php)
$path = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);
if ($path !== '/' && file_exists(__DIR__ . $path)) {
    return false;
}
require __DIR__ . '/index.php';

Start the server like this:

php -S localhost:8000 router.php

On a production server (Apache or Nginx) this is handled with .htaccess or server configuration, so router.php is only needed for local testing.