rewrite htaccess for specific uRL?

I have some domains that all point to the same backend folder. However, I would like to keep the URL the same.
For example, if someone accesses mysite.com or mysite.eu or mysite.net they will remain within that domain the entire time.

ANy idea show to change this and will that mess up any sessions?

Code:

---

RewriteEngine On
RewriteCond %{HTTP_HOST} !^(www\.mysite\.co\.uk)?$
RewriteCond $1 !^cart/updateorders\.php$
RewriteCond $1 !^IntegrityCheck\.php$
RewriteRule ^(.*)$ http://www.mysite.co.uk/$1 [R=302,L]

---

I also have this as the index.php page but I'm not sure if the htaccess bypasses it? I was told it was better to have this for Google listings.

PHP Code:

---

<?php
header( "HTTP/1.1 302 Temporarily Moved" );
header('Location: [url]http://www.mysite.co.uk/cart/index.php');[/url]
?>

---

I don't believe this is possible since session data is tied to domains.