Redirect a subdomain to the main domain

[snoopy74]

Code:

RewriteEngine on
RewriteRule ^(.*)$ http://test1.com/$1 [P]

returns an Error 500.

btw: Wouldn't that way not disturb my URL / request host ?

Another form of my question:
Is it possible to use the sources from "www.test1.com" for all requests for "sub.test1.com" without visible changes in the URL ?

My customer installed WordPress and changed it for multi-domain-support (Doc from http://www.yourhelpcenter.de/2009/03/wor...tallation/). Now he did not want to install WP on all subdomains and support all those copies.