sub recv_from_client_at_front_edge() {
    unset req.http.x-use-engine;
    if req.http.Cookie ~ "f2b31d03ab7" {
        set req.http.x-use-engine = "php7";
    } else {
        set req.http.x-use-engine = "hhvm";
    }
}

sub recv_response_headers_from_mediawiki() {
    resp.http.Vary += "x-use-engine";
}

17:01 < bblack> the straw pseudo-code still seems like a legit approach to me, but in practice there's some rough edges to it.  chiefly, that cross my mind immediately:
17:02 < bblack> 1) Integrating it properly with the Cookie vs X-Orig-Cookie or whatever we do for general cookie-hacking on cache_text (it may be we need to look at 
                X-Orig-Cookie, depending on where this is all injected at)
17:03 < bblack> 2) We probably want to handle the Vary modification with more grace, as the approach there will effectively invalidate all cache contents on deploy.  
                Probably the right answer is to only use the special header (x-use-engine) for the minority (php7) case.