How to add $_SESSION variable to all links?

With certain inbound links, we have an id appended to the URL, like http://www.mysite.com/page1?keyword, where 'keyword' is then captured in the template.php file into a $_SESSION variable. We then want to append that variable onto all other pages that are then visited, as http://www.mysite.com/about-us?keyword.

How can I use the $fragment of the l() or url() functions to force all displayed links to have the $_SESSION variable appended to it?

Thank you for all the help. -Greg