An example usage for Google Cloud Vision API with PHP on GCE

composer.json

{
    "require": {
        "google/apiclient": "2.x-dev",
        "google/appengine-php-sdk": "^1.9"
    }
}

VisionService.php

<?php
require './vendor/autoload.php';

use Google_Client;
use Google_Service_Vision;
use Google_Service_Vision_AnnotateImageRequest;
use Google_Service_Vision_BatchAnnotateImagesRequest;
use Google_Service_Vision_Feature;
use Google_Service_Vision_Image;
use Google_Service_Vision_ImageSource;

$client = new Google_Client();
$client->useApplicationDefaultCredentials();
$client->setScopes([Google_Service_Vision::CLOUD_PLATFORM]);
$service = new Google_Service_Vision($client);
$body = new Google_Service_Vision_BatchAnnotateImagesRequest();
$feature = new Google_Service_Vision_Feature();
$feature->setType('LABEL_DETECTION');
$feature->setMaxResults(100);
$src = new Google_Service_Vision_ImageSource();
$src->setGcsImageUri($gcsurl);
$image = new Google_Service_Vision_Image();
$image->setSource($src);
$payload = new Google_Service_Vision_AnnotateImageRequest();
$payload->setFeatures($features);
$payload->setImage($image);
$body->setRequests([$payload]);
/** @var $res \Google_Service_Vision_BatchAnnotateImagesResponse */
$res = $service->images->annotate($body, $optParams);

Thank you very much! this is very informative.
I think you have a typo in line 25 I think it should be $feature (in singular)
Also, I try to reproduce this script in my platform (Laravel + google/apiclient) and I keep gettign this error:

Error calling POST https://vision.googleapis.com/v1/images:annotate: (403) Request had insufficient authentication scopes.

Do you have any idea why?