How to fetch auth token for service account using google API? - php

Question

I have a Wordpress site (running PHP) that needs to display its own analytics data to visitors using the google-api. I want to create the charts in Javascript, so I need to fetch an auth token from the PHP code and then pass that to the Javascript code. I can't figure out how to get the auth token.
So far, I have code similar to this working using my service account:
https://github.com/google/google-api-php-client/blob/master/examples/service-account.php
My code:
$client = new Google_Client();
$client->setAuthConfig($credentialsFilePath);
$client->addScope('https://www.googleapis.com/auth/analytics.readonly');
$client->setApplicationName("GoogleAnalytics");
$analytics = new Google_Service_Analytics($client);
$ga = $analytics->data_ga;
$start = date('Y-m-d', strtotime('-7 days'));
$end = date('Y-m-d');
$views = $ga->get('ga:'.$myAnalyticsId,
$start,
$end,
'ga:pageviews,ga:sessions,ga:newUsers',
array(
'dimensions' => 'ga:date',
'sort' => 'ga:date'
));
This all works, and I'm able to connect to Google_Service_Analytics and fetch analytics data. However, I can't figure out how to fetch a service access token using my credentials, that I can hand off to the Javascript code so I can use the Google Analytics API from Javascript.
This doesn't work:
$token = $client->getAccessToken();
Token just ends up being null. What do I need to do to fetch a token?

Answer 1

Here is the working code for me.
The service account has a different type of process for generating access token.
It is not following oauth client api.
//include_once 'vendor/autoload.php';
$credentialsFilePath = 'test.json';
$client = new Google_Client();
$client->setAuthConfig($credentialsFilePath);
$client->addScope('https://www.googleapis.com/auth/analytics.readonly');
$client->setApplicationName("GoogleAnalytics");
$client->refreshTokenWithAssertion();
$token = $client->getAccessToken();
print_r($token);
$accessToken = $token['access_token'];

Answer 2

Figured this out:
$client->refreshTokenWithAssertion();
$token = $client->getAccessToken();
$accessToken = $token['access_token'];

Answer 3

First answer still works in 2021 (Thanks!), but a used function is deprecated (still available though)
This worked for me
public function getToken() {
$client = new Client(); // use Google\Client;
$client->setAuthConfig('securejsonfile.json');
$client->addScope('https://www.googleapis.com/auth/cloud-platform');
$client->setApplicationName('appname');
$client->fetchAccessTokenWithAssertion();
return $client->getAccessToken(); //array
}