function user_os($userAgent) {
    $oses = array(
        'iPhone' => '(iPhone)',
        'Windows 3.11' => 'Win16',
        'Windows 95' => '(Windows 95)|(Win95)|(Windows_95)',
        'Windows 98' => '(Windows 98)|(Win98)',
        'Windows 2000' => '(Windows NT 5.0)|(Windows 2000)',
        'Windows XP' => '(Windows NT 5.1)|(Windows XP)',
        'Windows 2003' => '(Windows NT 5.2)',
        'Windows Vista' => '(Windows NT 6.0)|(Windows Vista)',
        'Windows 7' => '(Windows NT 6.1)|(Windows 7)',
        'Windows NT 4.0' => '(Windows NT 4.0)|(WinNT4.0)|(WinNT)|(Windows NT)',
        'Windows ME' => 'Windows ME',
        'Open BSD' => 'OpenBSD',
        'Sun OS' => 'SunOS',
        'Linux' => '(Linux)|(X11)',
        'Safari' => '(Safari)',
        'Mac OS' => '(Mac_PowerPC)|(Macintosh)',
        'QNX' => 'QNX',
        'BeOS' => 'BeOS',
        'OS/2' => 'OS/2',
        'Search Bot' => '(nuhk)|(Googlebot)|(Yammybot)|(Openbot)|(Slurp/cat)|(msnbot)|(ia_archiver)'
    );
    foreach ($oses as $os => $pattern) {
        if (eregi($pattern, $userAgent)) {
            return $os;
        }
    }
    return 'Unknown';
}