You can use custom runtimes to add
additional functionality to a PHP app running in the App Engine flexible environment. To configure
a custom runtime, replace the following line in your app.yaml file:
runtime: php
with this line:
runtime: custom
You must also specify a base image
by adding a Dockerfile in the same directory that contains the app.yaml file.
Visit the Custom runtimes documentation to learn how to define a Dockerfile in a custom runtime.
Health checking
By default, all programs running in the flexible environment receive
health check requests.
The base image gcr.io/google_appengine/php responds to the health check
request, so you don't have to write your own health check handler. If you're
using a different image, you need to disable health checks, or write your own
code to handle health check requests.