Instead of a redirect, I'd include the seperate indexes. This way you don't lose the semblance that you are running two sites.
switch ($_HOST){
case: "pornsite";
include ('pornindex.php');
break;
default:
include('movieindex.php');
break;
}
This is also a better method because on lower versions of Netscape, header("location: "); doesn't work.
This is what I did until I found a host that allowed pointed domains, which gives you the ability to run a whole site from a subdirectory.