How to Protect Firebase Credentials

I have an application in php that consumes the api of the firebase service account. I use the credential.json file provided by firebase.

I wonder how I can protect this file from improper access? The code below is from the class that makes the service available. The credential is in the same folder.

namespace App\Services\Google\Firebase;
use Kreait\Firebase\Factory; use Kreait\Firebase\ServiceAccount;

class FirebaseService {

public $database;
public $firebase;

public function __construct() {
// This assumes that you have placed the Firebase credentials in the same 
directory
// as this PHP file.
$serviceAccount = ServiceAccount::fromJsonFile(__DIR__ . '/credencial.json');

$this->firebase = (new Factory)
        ->withServiceAccount($serviceAccount)
        // The following line is optional if the project id in your credentials file
        // is identical to the subdomain of your Firebase project. If you need it,
        // make sure to replace the URL with the URL of your project.
        ->withDatabaseUri('url...')
        ->create();

$this->database = $this->firebase->getDatabase();
}
}