Add code sample for users API.

Docs live at https://cloud.google.com/appengine/docs/php/users/

Author: SurferJeffAtGoogle

appengine/standard/users/README.md

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+# Users & Google App Engine
+
+This sample application demonstrates how to use Google App Engine's users API.
+
+## Prerequisites
+
+- Install [`composer`](https://getcomposer.org)
+- Install dependencies by running:
+
+```sh
+composer install
+```
+
+## Deploy to App Engine
+
+**Prerequisites**
+
+- Install the [Google Cloud SDK](https://developers.google.com/cloud/sdk/).
+
+**Deploy with gcloud**
+
+```
+gcloud config set project YOUR_PROJECT_ID
+gcloud preview app deploy
+gcloud preview app browse
+```
+
+The last command will open `https://{YOUR_PROJECT_ID}.appspot.com/`
+in your browser.

appengine/standard/users/app.php

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+<?php
+/**
+ * Copyright 2015 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+# [START get_current_user]
+use google\appengine\api\users\User;
+use google\appengine\api\users\UserService;
+use Silex\Application;
+
+// create the Silex application
+$app = new Application();
+
+$app->get('/', function () use ($app) {
+    $user = UserService::getCurrentUser();
+
+    if (isset($user)) {
+        return sprintf('Welcome, %s! (<a href="%s">sign out</a>)',
+            $user->getNickname(),
+            UserService::createLogoutUrl('/'));
+    } else {
+        return sprintf('<a href="%s">Sign in or register</a>',
+            UserService::createLoginUrl('/'));
+    }
+});
+# [END get_current_user]
+
+return $app;

appengine/standard/users/app.yaml

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+runtime: php55
+threadsafe: yes
+api_version: 1
+
+handlers:
+- url: .*
+  script: index.php

appengine/standard/users/composer.json

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+{
+    "require": {
+        "silex/silex": "^1.3",
+        "php-http/guzzle6-adapter": "^1.0",
+        "google/apiclient": "^1.1"
+    },
+    "require-dev": {
+        "symfony/browser-kit": "^3.0"
+    }
+}