Firestore PHP Client permission denied

I want to use Google's Firestore to save user data, while the backend uses PHP. So far I configured the access rules like this:

rules_version = '2';

service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write: if request.auth != null;
    }
  }
}

I followed google's official documentation and my code looks like this:

$firestore = new FirestoreClient([
  'projectId' => env('firebase.project.id'),
  'keyfileName' => env('firebase.key.file')
]);

$collection = $firestore->collection('users');
$document = $collection->document($uid);

//this one triggers the permission error
$secret_key = $document->snaphot->get('secret_key');

Then I get:

PERMISSION_DENIED: Missing or insufficient permissions.

I tried to modify my rules like this:

rules_version = '2';

service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write: if request.auth.uid != null;
    }
  }
}

No difference. By the way, the firebase authentication part works, I can create users and make them logged in. Furthermore, if I turn of the rules the error vanishes, but obviosly that is not a solution. I'm using CodeIgniter 4.5.5 with the most recent google/cloud-firestore. When I will be back in my office, I can link all the dependencies. For now, I would appreciate any help.

Finally here is the solution.

I used kreait/firebase-php implementation of firestore. However make sure you fulfill all requirements listed in google's doc to make it work.

Dependencies in composer.json:

"firebase/php-jwt": "^6.10",
"google/cloud-firestore": "^1.47",
"google/gax": "1.35.1",
"google/protobuf": "^4.29",
"grpc/grpc": "^1.57",
"kreait/firebase-php": "^7.0"

Firestore:

use Google\Cloud\Firestore\DocumentSnapshot;
use Google\Cloud\Firestore\FirestoreClient;
use Kreait\Firebase\Contract\Auth;
use Kreait\Firebase\Factory;

...
 
    public function __construct()
    {
        $this->factory =  (new Factory())
            ->withServiceAccount(env('firebase.key.file'))
            ->withFirestoreDatabase('(default)');
 
        $this->auth = $this->factory->createAuth();

        //this one gives the FirestoreClient
        $this->firestore = $this->factory->createFirestore()->database();
    }

   ...
 
    public function getUserData(string $uid) : DocumentSnapshot
    {
        $collection = $this->firestore->collection('users');
        $user = $collection->document($uid);
        return $user->snapshot();
    }

    public function saveUserData(string $uid, array $data) : void
    {
        $collection = $this->firestore->collection('users');
        $user = $collection->document($uid);
        $user->set($data);
    }