Interacting with Google API's using their PHP libraries is a nightmare. Skip the headache by authorizing a Guzzle client and make raw HTTP requests instead!

simple-google-apis.php

<?php
$key_file_path = __DIR__ . '/service-account-credentials.json';

$google_client = new \Google_Client();
$google_client->setAuthConfig( $key_file_path );
// Set the scopes of whatever you need access to
// See https://developers.google.com/identity/protocols/oauth2/scopes
$google_client->setScopes( array( 'https://www.googleapis.com/auth/analytics.readonly' ) );
$http_client = $client->authorize();

// Now you have a Guzzle http client authorized to make requests to Google
$body_args = array(
	'reportRequests' => array(
		'viewId'     => 'xxxxxx',
		'dateRanges' => array(
			array(
				'startDate' => '7DaysAgo',
				'endDate'   => 'today',
			),
		),
		'metrics'    => array(
			array(
				'expression' => 'ga:sessions',
			),
		),
	),
);
$args      = array(
	'body' => json_encode( $body_args ),
);
$response  = $ga->request( 'POST', 'https://analyticsreporting.googleapis.com/v4/reports:batchGet', $args );
$raw_json  = (string) $response->getBody();
$json      = json_decode( $raw_json );

// Admire your glorious reward!
var_dump( $json );


// More Documention!!!
// Google Analytics reports.batchGet - https://developers.google.com/analytics/devguides/reporting/core/v4/rest/v4/reports/batchGet
// Guzzle Request Options           - https://docs.guzzlephp.org/en/stable/request-options.html