You can interpret CSS files with PHP (if you're not too bothered about server load), if you put this line in your Apache configuration:
addhandler application/x-httpd-php .css
Then you can try to rely on $_SERVER['HTTP_USER_AGENT']
, but beware it is not guaranteed to be trustworthy.
However, there are (IMO) better ways to achieve this, which include using IE's conditional comments:
<!--[if IE 8]>
<style type="text/css" ... />
<![endif]-->
Of course the most preferable way would be to rewrite your styles to avoid browser implementation differences, which is usually possible, but requires some level of CSS guru-ness if you're being forced to support an old browser like IE 6.