Changing referer URL

As many of you probably know the HTTP_REFERER environment variable gives the URL of the page that the user clicked on to come to your webpage.

I'd like to change this a bit however as when the user clicks on a link from a mirror of my main site I want them to think that they have been referred from my main site rather than the mirror.

Any ideas how to do this?

I doubt it. The user-agent (browser) does this.