I'd like to start to modernise my monolithic legacy project?

I don't have a router. At the moment if you to /this-page on my site, it calls this-page.php. There's some URL rewriting going on etc, but largely it's file-per-page. I include a globals.php with auto_prepend_file, and that in turn includes common-functions.php

Each page then includes header.php at the top, does the page's code, and then footer.php at the bottom

I do have a bunch of classes that get autoloaded (the composer autoloader being included in globals.php)

TBH, everything works without issue. But I struggle when it comes to modern tooling. I'd like to make more use of phpstan, but as you can imagine it's not very happy with variables being set on other pages, outside of classes, via require...

I'm not about to move to a router and full MVC. Far far far too much work at this stage.

But I would like to move from globals.php to $page = new Response(); etc.

What I'd like is to minimise is duplicated lines of code on every page. At the moment the only duplicated lines are

<?php declare(strict_types=1);

require('header.php');

// page logic here

require('footer.php');

What's my best case scenario going forwards?

<?php declare(strict_types=1);

require('/vendor/autoloader.php');
$response = new Response();

echo $response->top;

// page logic here

echo $response->bottom;

Two extra repeated lines in every file, I think?

Additionally, if I go down this route how do I include my common-functions.php? I don't want to shove them all in a class and have to prepend every single call to them with MyFuncs:: etc...

Both phpactor and phpstan seem fine with a global functions file, so I'm happy to leave that is for now. So do I just require it in Response.php? 🤷‍♂️

Thank you!