When I try to deploy my PHP application to Google App Engine, why am I suddenly getting a 400 error?

Question

Running the deploy command:

appcfg.py update ./appengine-project

I started seeing this error:

Error 400: --- begin server output ---
PHP 5.4 applications are prevented from being deployed to Google App
Engine from any version of the SDK, including older ones. If you need
to continue to deploy PHP 5.4 applications for compatibility reasons, 
you can request that your application be whitelisted for PHP 5.4
deployment by visiting http://goo.gl/qjKEuk.
--- end server output ---

I am running PHP 5.5 locally. And I deployed my application just a couple days ago without issue.

Answer 1

This Google Groups post here got me sorted out.

In my app.yaml file, I just needed to change this line:

runtime: php

to:

runtime: php55

After that, the deploy succeeded.