Google Server-to-Server OAuth2 PHP example not working - php

Question

I am following this example to get server-to-server access working between my PHP application and Google Cloud API's,
https://developers.google.com/api-client-library/php/auth/service-accounts. In particular, I need to access the Drive API.
When I run the application though I get the following error:
Google_Service_Exception : {
"error": "unauthorized_client",
"error_description": "Client is unauthorized to retrieve access tokens using this method, or client not authorized for any of the scopes requested."
}
Here's what I have done:
Created a GCP project
Added a service account to the project, with domain wide delegation, and a set of keys
Enabled the Google Drive API in the project
In my G Suite account, under 'Manage API client access', I have added the Client ID of my service account, with the permission of https://www.googleapis.com/auth/drive
Have I missed a step?
Here's my code:
putenv('GOOGLE_APPLICATION_CREDENTIALS=my_storage/secure/my-app-service-account.json');
$client = new \Google_Client();
$client->setScopes(\Google_Service_Drive::DRIVE_METADATA_READONLY);
$client->useApplicationDefaultCredentials();
$client->setSubject('my_email#my_domain.com');
$drive = new \Google_Service_Drive($client);
echo $drive->about->get();
The example does not include the setScopes() call, however I was getting a scope-related error without it. (The example is not based on the Drive API, so perhaps it's not required in that case?)
UPDATE: In the IAM settings for the GCP, I added the email address of the service account as a Project Owner, but that made no difference.

Answer 1

I found the problem.
In the setScopes() call above, the value passed needs to be the same as the value given when setting up the client in G Suite, in this case https://www.googleapis.com/auth/drive