Support Apache environment variables

[jeffkaufman]

In Nginx we support setting script variables; in Apache this would be useful as well. For example, see https://groups.google.com/d/msgid/mod-pagespeed-discuss/CAHRWggQSZ-0Nf6VqvRrRXLw3EYWxO8kegXqErG-%3DYf9FVgDshw%40mail.gmail.com?utm_medium=email&utm_source=footer

Apache doc: https://httpd.apache.org/docs/2.2/env.html

[morlovich]

When do the nginx ones evaluate?

[jeffkaufman]

Nginx ones are parsed and "compiled" when we load the configuration, and then evaluated at request time.

[sirshurf]

Subscribing myself as this is something I really need (I was the poster of #1175).

[rexmac]

My team has recently implemented experiments via ModPagespeedRunExperiment, but we're struggling to come up with a way of specifying different GA web property IDs for different environments. We think that support for env variables would be very helpful in this regard so that we could do something akin to:

<If env('SERVER_ENV') == 'production'>
    ModPagespeedAnalyticsID UA-XXXXXXX-Y
</If>
<Else>
    ModPagespeedAnalyticsID UA-XXXXXXX-Z
</Else>

[phuib]

This would be great for setting up many different websites without having to hard code everything into the apache configs.