New-ish to PHP

[mralls8]

I'm doing ok getting the shopping site set up, but I'm semi-new to PHP and here's my problem:

 

My main site is in xhtml, not php. From my main site I want to link to the individual category pages, not to the catalog index.php

 

Does that make any sense? For instance if I'm selling CDs, Movies, and Electronics, I want links from my xhtml index page to all of those categories, one to the CD shopping page, and so on.

 

I'm not sure how to link to them, and more importantly, how to tell the link to get the SID.

 

Help?