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.
As of PHP 5.5, the MySQL library has been officially deprecated. It is recommended to move to either MySQLi or PDO libraries for your mysql connectivity. See here for help choosing which interface you prefer: http://php.net/manual/en/mysqlinfo.api.choosing.php