$user_agent = strtolower ( $_SERVER['HTTP_USER_AGENT'] ); 
    if ( preg_match ( "/phone|iphone|itouch|ipod|symbian|android|htc_|htc-|palmos|blackberry|opera mini|iemobile|windows ce|nokia|fennec|hiptop|kindle|mot |mobile|pda;|avantgo|eudoraweb|minimo|netfront|brew|teleca|lg;|lge |wap;| wap |mot-|webos\/|samsung|sonyericsson|^sie-|nintendo/", $user_agent ) ) { 
        echo 'mobile.css'; 
    } else { 
        echo 'style.css'; 
    }