Vercel PHP | Not finding Files

I currently have a PHP project that I'm transferring over to Vercel (for the free hosting). I'm struggling to load any files outside of the index.php file, I've created a simpler project for the purposes of this question, however I'm still getting the same errors.

This is my file structure:

MyPHPProject
    - index.php
    - vercel.json
    - api
        - index.php
    - another-page
        - index.php

Below is my vercel.json file:

{
    "functions": {
        "api/*.php": {
            "runtime": "vercel-php@0.4.0"
        }
    },
    "routes": [
        { "src": "/(.*)",  "dest": "/api/index.php" }
      ]
}

This is my api/index.php file:

<?php include __DIR__ . '/../index.php'; ?>

This is my ```index.php`` file (in main route):

<html>
    <head></head>

<body>

<div class="testbackground">
    <a href="/another-page">Another Page</a>

    <h1>Hello there!</h1>

    <?php include __DIR__ . '/another-page/index.php'; ?>
</div>
</body>
</html>

My another-page/index.php file is:

<h1>This better work</h1>

When I go to my server (https://testingphp.vercel.app/) I get the following:

Another Page
Hello there!
This better work

However when I go to https://testingphp.vercel.app/another-page, it doesn't load the correct index.php file.

Any help is much appreciated.