Google API Oauth Access Token (Laravel PHP)

GoogleOauthHelper.php

<?php

# composer require firebase/php-jwt

use Firebase\JWT\JWT;
use Illuminate\Support\Facades\Http;


class GoogleOauthHelper
{ 
    /**
     * JSON file downloaded when creating Google Cloud Service Account 
     * at https://console.cloud.google.com/apis/credentials?project=
     * 
     * @var string
     */
    protected $credentialFilePath;
  
     /**
     * 
     */
    public function __construct()
    {
        // eg: "google_service_account.json" if located at base path
        $this->credentialFilePath = config('services.google.credential_file_path'); 
    }


    /**
     * Create OAuth JWT Token
     * Maximum validity is 1 Hour
     * 
     */
    private function createJWTToken($validSeconds = 3600)
    {
        if($validSeconds > 3600) {
            throw new Exception('Valid JWT period cannot more than 1 Hour.');
        }

        $iat = time();
        $exp = $iat + $validSeconds;
      
        $content = file_get_contents($this->credentialFilePath);
        $jsonObj = json_decode($content);
      
        $payload = [
            "iss" => $jsonObj->client_email,    // eg: "xxxxxxxxxxxxxxxxx.iam.gserviceaccount.com",
            "aud" => $jsonObj->token_uri,       // eg: "https://oauth2.googleapis.com/token"
            "scope" => "xxxxxxxxxxxxx"          // eg: 'https://www.googleapis.com/auth/firebase.messaging' for Cloud Messaging
            'iat' => $iat,
            'exp' => $exp
        ];

        return  JWT::encode(
                    $payload, 
                    $jsonObj->private_key,      // eg: "-----BEGIN PRIVATE KEY----- .........",
                    "RS256"
                );
    }


    /**
     * Fetch API access token
     */
    public function getAccessToken(): string
    {
        $jwtToken = $this->createJWTToken();

        $res =  Http::asForm()
                ->post('https://oauth2.googleapis.com/token', [
                    'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
                    'assertion' => $jwtToken
                ]);

        if($res->successful()) {

            /**
             * Sometimes, trailing dots returned, need trim
             * https://stackoverflow.com/questions/68654502/why-am-i-getting-a-jwt-with-a-bunch-of-periods-dots-back-from-google-oauth
             * 
             */
            $body = $res->json();

            return rtrim($body["access_token"], '.');
        }

        throw new Exception('Failed to fetch access token. Body: ' . $res->body());
    }
}

SomeController.php

<?php

namespace App\Http\Controllers;

use App\Helpers\GoogleOauthHelper;


class SomeController extends Controller
{  
    $helper = new GoogleOauthHelper;
  
    return $helper->getAccessToken();
}