How do I get PHP code to be executed within my CSS files _AND_ have it work when the page hass XHTML Transitional or XHTML Strict DocTypes?

Right now I set my stylesheets to be a php file (styles.php) and I have my page's DocType as XHTML Strict. It works fine in IE 6 and Opera 7.5 but it fails in FireFox (no styles are shown). Same thing happens if I change the page to XHTML Transitional. When I remove the DocType, however, it works fine. Any ideas?

I've also tried adding the following to my .htacess file but this also fails (the php codes doesn't get executed).

Code:
AddType application/x-httpd-php .css
And to mods: I posted this in the CSS forum because this really has more to do with CSS than PHP.