Using OAuth 2.0 for Server to Server Applications

The Google APIs Client Library for PHP supports using OAuth 2.0 for server-to-server interactions such as those between a web application and a Google service. For this scenario you need a service account, which is an account that belongs to your application instead of to an individual end user. Your application calls Google APIs on behalf of the service account, so users aren't directly involved. This scenario is sometimes called "two-legged OAuth," or "2LO." (The related term "three-legged OAuth" refers to scenarios in which your application calls Google APIs on behalf of end users, and in which user consent is sometimes required.)

Typically, an application uses a service account when the application uses Google APIs to work with its own data rather than a user's data. For example, an application that uses Google Cloud Datastore for data persistence would use a service account to authenticate its calls to the Google Cloud Datastore API.

If you have a Google Apps domain—if you use Google Apps for Work, for example—an administrator of the Google Apps domain can authorize an application to access user data on behalf of users in the Google Apps domain. For example, an application that uses the Google Calendar API to add events to the calendars of all users in a Google Apps domain would use a service account to access the Google Calendar API on behalf of users. Authorizing a service account to access data on behalf of users in a domain is sometimes referred to as "delegating domain-wide authority" to a service account.

This document describes how an application can complete the server-to-server OAuth 2.0 flow by using the Google APIs Client Library for PHP.

  1. Overview
  2. Creating a service account
    1. Delegating domain-wide authority to the service account
  3. Preparing to make an authorized API call
  4. Calling Google APIs
  5. Complete example

Overview

To support server-to-server interactions, first create a service account for your project in the API Console. If you want to access user data for users in your Google Apps domain, then delegate domain-wide access to the service account.

Then, your application prepares to make authorized API calls by using the service account's credentials to request an access token from the OAuth 2.0 auth server.

Finally, your application can use the access token to call Google APIs.

Creating a service account

A service account's credentials include a generated email address that is unique, a client ID, and at least one public/private key pair.

If your application runs on Google App Engine, a service account is set up automatically when you create your project.

If your application doesn't run on Google App Engine or Google Compute Engine, you must obtain these credentials in the Google API Console. To generate service-account credentials, or to view the public credentials that you've already generated, do the following:

  1. Open the Service accounts page. If prompted, select a project.
  2. Click Create service account.
  3. In the Create service account window, type a name for the service account, and select Furnish a new private key. If you want to grant G Suite domain-wide authority to the service account, also select Enable G Suite Domain-wide Delegation. Then click Create.

Your new public/private key pair is generated and downloaded to your machine; it serves as the only copy of this key. You are responsible for storing it securely.

You can return to the API Console at any time to view the client ID, email address, and public key fingerprints, or to generate additional public/private key pairs. For more details about service account credentials in the API Console, see Service accounts in the API Console help file.

Take note of the service account's email address and store the service account's private key file in a location accessible to your application. Your application needs them to make authorized API calls.

Delegating domain-wide authority to the service account

If your application runs in a Google Apps domain and accesses user data, the service account that you created needs to be granted access to the user data that you want to access.

The following steps must be performed by an administrator of the Google Apps domain:

  1. Go to your Google Apps domain’s Admin console.
  2. Select Security from the list of controls. If you don't see Security listed, select More controls from the gray bar at the bottom of the page, then select Security from the list of controls. If you can't see the controls, make sure you're signed in as an administrator for the domain.
  3. Select Advanced settings from the list of options.
  4. Select Manage third party OAuth Client access in the Authentication section.
  5. In the Client name field enter the service account's Client ID.
  6. In the One or More API Scopes field enter the list of scopes that your application should be granted access to. For example, if your application needs domain-wide access to the Google Drive API and the Google Calendar API, enter: https://www.googleapis.com/auth/drive, https://www.googleapis.com/auth/calendar.
  7. Click Authorize.

Your application now has the authority to make API calls as users in your domain (to "impersonate" users). When you prepare to make authorized API calls, you specify the user to impersonate.

Preparing to make an authorized API call

After you have obtained the client email address and private key from the API Console, set the path to these credentials in the GOOGLE_APPLICATION_CREDENTIALS environment variable ( Note: This is not required in the App Engine environment):

putenv('GOOGLE_APPLICATION_CREDENTIALS=/path/to/service-account.json');

Call the useApplicationDefaultCredentials to use your service account credentials to authenticate:

$client = new Google_Client();
$client->useApplicationDefaultCredentials();

If you have delegated domain-wide access to the service account and you want to impersonate a user account, specify the email address of the user account using the method setSubject:

$client->setSubject($user_to_impersonate);

Use the authorized Google_Client object to call Google APIs in your application.

Calling Google APIs

Use the authorized Google_Client object to call Google APIs by completing the following steps:

  1. Build a service object for the API that you want to call, providing the authorized Google_Client object. For example, to call the Cloud SQL Administration API:
    $sqladmin = new Google_Service_SQLAdmin($client);
  2. Make requests to the API service using the interface provided by the service object. For example, to list the instances of Cloud SQL databases in the examinable-example-123 project:
    $response = $sqladmin->instances->listInstances('examinable-example-123')->getItems();

Complete example

The following example prints a JSON-formatted list of Cloud SQL instances in a project.

To run this example:

  1. Create a new directory and change to it. For example:
    mkdir ~/php-oauth2-example
    cd ~/php-oauth2-example
  2. Install the Google API Client Library for PHP using Composer:
    composer require google/apiclient:^2.0
  3. Create the file sqlinstances.php with the content below.
  4. Run the example from the command line:
    php ~/php-oauth2-example/sqlinstances.php

sqlinstances.php

<?php

require_once __DIR__.'/vendor/autoload.php';

putenv('GOOGLE_APPLICATION_CREDENTIALS=/path/to/service-account.json');
$client = new Google_Client();
$client->useApplicationDefaultCredentials();

$sqladmin = new Google_Service_SQLAdmin($client);
$response = $sqladmin->instances
    ->listInstances('examinable-example-123')->getItems();
echo json_encode($response) . "\n";