[exim] No Local Deliveries

In Exim's config file how do I tell Exim to do the following:

1. Treat all deliveries as a remote delivery even if the domain is listed in
local_domains
2. Never try to deliver email locally - ever!
3. Only ever relay mail from the localhost

e.g. A web hosting client has a php script using the mail() and mail is
being piped to Exim on the same server to handle smtp.