Нужно пару функций

function get_user_os($user_agent)
{
    $os_arr = array("Windows NT 4.0" => "Windows NT",
                    "Windows NT 3.5" => "Windows NT",
                    "Windows NT 5.0" => "Windows 2000",
                    "Windows NT 5.1" => "Windows XP",
                    "Windows NT 5.2" => "Windows Server 2003",
                    "Windows NT 6.0" => "Windows Vista",
                    "Windows NT 6.1" => "Windows 7",
                    "Windows CE" => "Windows CE or Windows Mobile",
                    "Windows Me" => "Windows Me",
                    "Windows 98" => "Windows 98",
                    "Windows 95 " => "Windows 95",
                    "Linux" => "Linux",
                    "Lynx" => "Linux",
                    "Unix" => "Linux",
                    "Macintosh" => "Macintosh",
                    "PowerPC" => "Macintosh",
                    "OS/2" => "OS/2",
                    "BeOS" => "BeOS" );

    foreach ( $os_arr as $key => $value )
    {
        if ( !strstr( strtolower( $user_agent ), strtolower( $key ) ) ){
            continue;
        }
            return $value;
    }
    
    return FALSE;
}
$useragent = $_SERVER["HTTP_USER_AGENT"];
$user_os = get_user_os($useragent);