@kev when you say "nginx sits in front of apache for cache" do you know for a fact that it caches?l and doesn't propagate all requests to apache (and php)? It's unusual to use nginx for that, I don't even know how (though I think it's possible)
Ideally for static content (including dynamic pages that can be cached for a while) you should not even hit php in the backend. Hence all the people shouting "varnish".