You can use [custom runtimes](https://docs.cloud.google.com/appengine/docs/flexible/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](https://docs.cloud.google.com/appengine/docs/flexible/reference/app-yaml): runtime: php with this line: runtime: custom You must also [specify a base image](https://docs.cloud.google.com/appengine/docs/flexible/custom-runtimes/build#base) by adding a Dockerfile in the same directory that contains the `app.yaml` file. Visit the [Custom runtimes](https://docs.cloud.google.com/appengine/docs/flexible/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](https://docs.cloud.google.com/appengine/docs/flexible/how-instances-are-managed). Make sure to write your own code to handle health check request, or disable health checks if needed.