google-cloud-storage-upload.php

<?php
require_once("src/Google/Client.php");
require_once("src/Google/Service/Storage.php");

/**
 * Connect to Google Cloud Storage API
 */
$client = new Google_Client();
$client->setApplicationName("App Name");

// $stored_access_token - your cached oauth access token 
if( $stored_access_token ) {
	$client->setAccessToken( $stored_access_token );
}

$credential = new Google_Auth_AssertionCredentials(
					"service account email address",
					['https://www.googleapis.com/auth/devstorage.read_write'],
					file_get_contents("path/to/private-key-certificate.p12")
				);

$client->setAssertionCredentials($credential);
if($client->getAuth()->isAccessTokenExpired()) {
	$client->getAuth()->refreshTokenWithAssertion($credential);
	// Cache the access token however you choose, getting the access token with $client->getAccessToken()
}

/**
 * Upload a file to google cloud storage
 */
$storage = new Google_Service_Storage($client);

$file_name = "what you want your file to be named on Google Cloud Storage";

$obj = new Google_Service_Storage_StorageObject();
$obj->setName($file_name);

$storage->objects->insert(
	"bucket name",
	$obj,
	['name' => $file_name, 'data' => file_get_contents("local path of the file to upload"), 'uploadType' => 'media']
);