Mod Rewrite - Issue with Add-on domain www.domain.com vs domain.com

Hey everyone,
Hope this isn't too basic, I'm having an issue with a mod rewrite of an add on domain hosted on Lunar Pages.

Because it is an add-on domain to my main hosting account, the add-on domain appears in a folder off of my domain root. Since I have a mod rewrite for the main domain (everything with a www), the rewrite is affecting the add-on domains creating an unrecognizable address.

The bad mod rewrite address looks something like this:
www.addondomain.com/addondomain.com

I've tried to edit my .htaccess as follows:

RewriteCond %{HTTP_HOST} !^www\.
RewriteCond %{HTTP_HOST} ^maindomain$ [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]

Code (markup):

My attempted logic above is to force the rewrite only with a match to maindomain...

Any thoughts on how to better achieve this? Thanks

 

RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^www\.maindomain\.com [NC]
RewriteRule (.*) http://www.maindomain.com/$1 [R=301,L]

Code (markup):

 

Actually, I'm getting the code to work now. I thought that my browser had been refreshed (looking for a new version of the page each time)... but maybe wasn't. Lol, of course now it seems to be working...

Thank you snatcher and everyone for your thoughts.

 

This has nothing to do with PHP

 

<IfModule mod_rewrite.c>
   RewriteEngine On
   RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
   RewriteRule ^(.*)$ http://%1/$1 [R=301,L]
</IfModule>

Code (markup):

this one will work best for all cases