How do I create a LOGOUT link in PHP

I have developed a few PHP pages that sit outside my Gallery pages. These pages use the Gallery embed functionality to allow users to login and register from these pages into the G2 application. This all works fine.

What I can't do, however, is to display a LOGOUT link on these PHP pages that sit outside G2. When I copy a logout link from my actual G2 pages and place it on my PHP pages it results in an "Authentication failure".

I'm sure there's an easy line of code to do this, so if anyone knows, I'd love to hear your advice.

Thanks in advance