Generate an Atom RSS feed from Google Play Store app reviews.

It uses Google's Reply to Reviews API to generate the feed based on apps you own. (It does not work with apps you don't control; this is Google's restriction.)

1. Download Composer if you don't have it already.
2. Run composer install to install the Google API client.
3. Follow Google's GPS API Getting Started Guide to set up a "service account." (Beware, Google's OAuth documentation is a labyrinth, and they change it every year.)

Here's what it says as of May 21, 2016:

1. Go to the API Access page on the Google Play Developer Console.
2. Under Service Accounts, click Create Service Account.
3. Follow the instructions on the page to create your service account.
4. Once you’ve created the service account on the Google Developers Console, click Done. The API Access page automatically refreshes, and your service account will be listed.
5. Click Grant Access to provide the service account the necessary rights to perform actions.

4. While setting up your service account, you'll download a JSON file containing your secret credentials. Save that file as client_secret.json where the PHP file can reach it.
   Make sure your users cannot access this file.
5. Open index.php in your RSS reader, with an URL parameter specifying the package you want to follow, e.g.:

index.php?package=com.example.app