I use PHP for a couple reasons. One being that i can make them easier to update. I'll set a variable for font size that i can echo in multiple places. Then i can Divide that number by 2 and use it for paddings.... stuff like that. Another reason is so i can use PHP to check browser type and version and conditionally output stuff. I also use a PHP class to handle colors. And finally I use php to stitch all my individual css files into one. That makes it faster loading and I can easily alter the configuration of a site with a master config file instead of unnecessarily outputting unused styles.
I HIGHLY recommend doing this if possible. Luckily i have 100% control of my server. Even if I can't get them to cache properly... it's worth it with the million headaches it solves.