adds slim framework example (GoogleCloudPlatform#659)

Author: bshaffer

appengine/php72/slim-framework/.gcloudignore

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+# This file specifies files that are *not* uploaded to Google Cloud Platform
+# using gcloud. It follows the same syntax as .gitignore, with the addition of
+# "#!include" directives (which insert the entries of the given .gitignore-style
+# file at that point).
+#
+# For more information, run:
+#   $ gcloud topic gcloudignore
+#
+.gcloudignore
+# If you would like to upload your .git directory, .gitignore file or files
+# from your .gitignore file, remove the corresponding line
+# below:
+.git
+.gitignore
+
+# PHP Composer dependencies:
+vendor/

appengine/php72/slim-framework/README.md

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+# Slim Example for App Engine for PHP 7.2
+
+This sample demonstrates how to deploy a *very* basic [Slim][slim] application to
+[Google App Engine for PHP 7.2][appengine-php].
+
+## Setup
+
+Before running this sample:
+
+### Create a project (if you haven't already)
+
+- Go to [Google Developers Console][console] and create a new project.
+
+## Deploy to App Engine
+
+**Deploy with gcloud**
+
+```
+gcloud config set project YOUR_PROJECT_ID
+gcloud app deploy
+gcloud app browse
+```
+
+The last command will open `https://{YOUR_PROJECT_ID}.appspot.com/`
+in your browser.
+
+## Application Components
+
+The application consists of three components:
+
+ 1. An [`app.yaml`](app.yaml) which sets your application runtime to be `php72`.
+ 2. A [`composer.json`](composer.json) which declares your application's dependencies.
+ 3. An [`index.php`](index.php) which handles all the requests which get routed to your app.
+
+The `index.php` file is the most important. All applications running on App Engine
+for PHP 7.2 require use of a [front controller][front-controller] file.
+
+[console]: https://console.developers.google.com/project
+[slim]: https://www.slimframework.com/
+[appengine-php]: https://cloud.google.com/appengine/docs/standard/php/
+[front-controller]: https://stackoverflow.com/questions/6890200/what-is-a-front-controller-and-how-is-it-implemented-in-php

appengine/php72/slim-framework/app.yaml

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+runtime: php72
+

appengine/php72/slim-framework/composer.json

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+{
+    "require": {
+        "slim/slim": " ^3.0"
+    },
+    "require-dev": {
+        "google/cloud-tools": "^0.8.1",
+        "phpunit/phpunit": "^5"
+    }
+}