Setting up Google Reporting API to work in cronjob - php

Question

I've managed to set up a PHP script that grabs data from our Google Analytics account and can run this great in the browser but I'm struggling to figure out how to allow this to work inside of the cronjob script instead.
Here's my code:
<?php
// Load the Google API PHP Client Library.
require_once dirname(__FILE__) . '/analytics/google-api-php-client-2.2.0/vendor/autoload.php';
session_start();
$client = new Google_Client();
$client->setAuthConfig(dirname(__FILE__) . '/analytics/client_secrets.json');
$client->setAccessType("offline"); //offline access
$client->addScope(Google_Service_Analytics::ANALYTICS_READONLY);
if (isset($_SESSION['access_token']) && $_SESSION['access_token']) {
print "<br />start: ".date("H:i:s")."<br />";
// Set the access token on the client.
$client->setAccessToken($_SESSION['access_token']);
$sessionid = $_SESSION['access_token']['access_token'];
// Create an authorized analytics service object.
$analytics = new Google_Service_AnalyticsReporting($client);
$view_id = urlencode("ga:XXXXXX");
$start_date = "yesterday"; //can be words or dates in Y-m-d
$end_date = "yesterday"; //can be words or dates in Y-m-d
$metrics = urlencode("ga:transactions");
$dimensions = urlencode("ga:transactionId,ga:date,ga:orderCouponCode");
$sort = "ga:date"; //for DESC use -FIELD e.g. -ga:date
$sampling = "HIGHER_PRECISION"; //https://developers.google.com/analytics/devguides/reporting/core/v3/reference#samplingLevel
$limit = "10000";
$filters = urlencode("ga:orderCouponCode!=(not set)");
$params = "ids=$view_id&start-date=$start_date&end-date=$end_date&metrics=$metrics&dimensions=$dimensions&filters=$filters&sort=$sort&samplingLevel=$sampling&max-results=$limit&access_token=$sessionid";
$data = file_get_contents("https://www.googleapis.com/analytics/v3/data/ga?$params");
$json_data = json_decode($data);
print "<pre>";
print_r($json_data);
print "</pre>";
} else {
// Handle authorization flow from the server.
if (! isset($_GET['code'])) {
$auth_url = $client->createAuthUrl();
header('Location: ' . filter_var($auth_url, FILTER_SANITIZE_URL));
} else {
$client->authenticate($_GET['code']);
$_SESSION['access_token'] = $client->getAccessToken();
$redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/cronjobs/ga_coupon_codes.php';
header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}
}
print "<br />end: ".date("H:i:s")."<br />";
When my cronjob runs, the servers send me an email to say that it has finished running. I'm expecting to see both the "start" and "end" times printed in the response, as well as the JSON data, but the only thing that's coming back in the "end" time, leading me to believe it's not running through the main bit of it.
I know the main reason why it's not working is due to the authentication bit. In a cronjob I cannot use header('location: URL') but I can't figure out what to change this to in order to make it work.
When I set up the account, it was created as a service account and I can see from within the API console that I have a service account key.
I've also got an OAuth 2.0 client ID (which is what is in the client screts JSON file). If I run this in my browser I'm getting the connection to work, even after the hour runs out (where it's getting a new access token).
I've googled around for this for quite some time and found that adding "offline access" should help but obviously I'm still using the header() function. I can't find examples for me to use that are understandable to me and I'd really like to get this working in a cronjob.