Cloud Firestore Authentication not working using PHP - php

Question

I am new to firebase. I am testing a web application using the Firestore Database and PHP. https://github.com/googleapis/google-cloud-php-firestore. I have created a database "cities" and want to fetch the data only for the login User. I have set a rule on the database
rules_version = '2';
service cloud.firestore {
match /databases/{database}/documents {
match /cities/{city} {
allow read, write: if request.auth != null
}
}
}
Now after login is successful using email and password, I want to fetch the population of a city. I have checked firebase auth and it is showing me logged in.
firebase.auth().onAuthStateChanged((user) => {
if (user) {
var uid = user.uid;
console.log("Logged In"+uid)
} else {
console.log("Please login");
}
});
I am using the following PHP code.
$firestore = new FirestoreClient([
'projectId' => 'XXXXXXXX',
]);
$collectionReference = $firestore->collection('cities');
$documentReference = $collectionReference->document('kolkata');
$snapshot = $documentReference->snapshot();
echo "Population is " . $snapshot['population'];
Now I am getting the following error:
Fatal error: Uncaught Google\Cloud\Core\Exception\ServiceException: {
"message": "Missing or insufficient permissions.", "code": 7,
"status": "PERMISSION_DENIED", "details": [] }
Now when I change the security rule to allow read and write for all users, data are fetching properly.
I have searched the document but have not found any suggestions about the error.
Also, when I am using the keyfile along with the project id, I can fetch the data, but there is another issue. The security rule is not working anymore. I can read the data without logging in.
How can I solve this issue. Thanks in advance and sorry for the bad English.
$firestore = new FirestoreClient([
'projectId' => 'XXXXXX',
'keyFile' => json_decode(file_get_contents(__DIR__.'/secret/XXXXXXXX.json'), true)
]);