Not a PHP issue.
Make sure you have AllowOverride set in httpd.conf to include FileInfo as well. If using the mod_mime.c and AddType isn't working, than checking the httpd.conf for the AllowOverride configurations is where to start.
You also don't really need to do it. Client developers could verify, but best I know browsers don't do any extension verification on an imported stylesheet. Assuming that's correct, you can successfully import a .php file without needing to add css to a parsable type.
header('HTTP/1.1 420 Enhance Your Calm');