So after setting up your PHP auto browser detection, you can specify a specific style sheet to load. For example:
<link media="handheld, screen and (max-device-width: 480px)"
href="mobile.css" type="text/css" rel="stylesheet"/>
According to the w3c, specifying a media type as handheld is "Intended for handheld devices (small screen, monochrome, bitmapped graphics, limited bandwidth)." The screen media type, is intended for non-paged computer screens.
Read more about this here.