example.php

<?php
function __construct () 
	{
		parent::__construct();

		// accessed via google developer console
		$client_email = '****@developer.gserviceaccount.com';
		$private_key = file_get_contents(__DIR__ .'/secure.p12');
		$scopes = array(\Google_Service_Analytics::ANALYTICS_READONLY);
		$user_to_impersonate = '****';

		$credentials = new \Google_Auth_AssertionCredentials(
		    $client_email,
		    $scopes,
		    $private_key,
		    'notasecret',                                 
		    'http://oauth.net/grant_type/jwt/1.0/bearer', 
		    $user_to_impersonate
		);


		$client = new \Google_Client();
		$client->setAssertionCredentials($credentials);

		if ($client->getAuth()->isAccessTokenExpired()) 
		  	$client->getAuth()->refreshTokenWithAssertion();

		// set reference for instance methods
		$this->client = &$client;
	}