Given we are going to run multiple versions of php in parallel, it might be smart to separate caches between them for the time being.

This can be accomplished by adding a Vary: header to our responses, and a corresponding faux header to the requests.

The way this is done is as follows:

• Set the faux request header in varnish if we have a cookie PHP_ENGINE
• Change the responses in ATS to add a Vary: header

This is similar to what we did in https://gerrit.wikimedia.org/r/c/operations/puppet/+/478680, with the added complication we now use ATS on the backend.