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).

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.