The 'silent redirection' aspect can be very important to the feeling of your visitors 'safety, and being in control' thank you for that elusive modrewrite code!
As for you Carrie, if you wanted the simple, non-invisible PHP code redirection, you make a .php file that goes like this:
<?php
// First step, decide using the domain name which
// directory you want to redirect to
$redirtothisfile="default_index.html";
if ($HTTP_SERVER_VARS['HTTP_HOST']==="dogs.com")
{$redirtothisfile="/dogs/dogindex.html";}
if ($HTTP_SERVER_VARS['HTTP_HOST'] ==="purbreddogs.com")
{$redirtothisfile="/dogs/purbreadindex.html";}
if ($HTTP_SERVER_VARS['HTTP_HOST'] ==="cats.com")
{$redirtothisfile="/cats/index.html";}
if (SOME_OTHER_FACTOR) {$redirtothisfile="SOMEFILE"}
// 2) now add the 'HTML HEADER' redirection
// see the PHP manual for header() here:
//
http://www.php.net/manual/en/function.header.php
header("Location: $redirtothisfile");
die;
?>
And that's it! The browser will redirect with this, the script will end, and you'll see the new page etc. . .