Im not sure if PHP can do this... but...

I want to check to see if there are external links on my pages and if there are, i want it to point to a framed page of mine with a url like this:

http://www.rapidfriends.com/external...w.vbforums.com

I just want to know how i can have it automaticly change the url from:
http://www.vbforums.com

to

http://www.rapidfriends.com/external...w.vbforums.com