Problem:

I have a website (mywebsite.com) that was, and partly is, indexed in google. Somebody pointed their own domain (theirsite.com) name to my server and DNS, so it resolves with my IP. Now, probably being an older domain, it outranks me in google, and the pages at my domain are starting to getting de-indexed (probably duplicate content or something). So, for example, my homepage got de-indexed, and their homepage (theirsite.com/) is indexed with my content/code/etc. The same is for other pages (theirsite.com/other/page.html is showing mysite.com/other/page.html)

Quick-fix: To quickly fix it, I have added few lines to my PHP code, checking for $_SERVER['HTTP_HOST'], and if different than my domain, redirects to my domain. It does the job, but to me it looks like a dirty solution.

Question: I could not find a way to have apache to do this job. I would prefer to find an apache/.htaccess solution to this problem (redirecting all traffic from domainA.com/(.*) to domainB.com/$1), is it possible in any way?

Thanks