Look in $_SERVER['HTTP_USER_AGENT'].
If the user submitted it (and their browser isn't faking it), then it will tell you what browser they are using.
Searching on the web should probably get you the User Agent strings of the more popular WAP browsers.
You could also check for Mozilla, Firefox, IE, Opera, Safari, and Netscape, and then redirect if it's not any of those. (Of course, then the main browser page might get overlooked by the honest webbots ...)