A collection of three useful HTTP_USER_AGENT functions for the client IP, OS and Browser

http_user_agent_functions.php

// function to get the client ip address
function get_ip() {
    $ipaddress = '';
    if ($_SERVER['HTTP_CLIENT_IP'])
        $ipaddress = $_SERVER['HTTP_CLIENT_IP'];
    else if($_SERVER['HTTP_X_FORWARDED_FOR'])
        $ipaddress = $_SERVER['HTTP_X_FORWARDED_FOR'];
    else if($_SERVER['HTTP_X_FORWARDED'])
        $ipaddress = $_SERVER['HTTP_X_FORWARDED'];
    else if($_SERVER['HTTP_FORWARDED_FOR'])
        $ipaddress = $_SERVER['HTTP_FORWARDED_FOR'];
    else if($_SERVER['HTTP_FORWARDED'])
        $ipaddress = $_SERVER['HTTP_FORWARDED'];
    else if($_SERVER['REMOTE_ADDR'])
        $ipaddress = $_SERVER['REMOTE_ADDR'];
    else
        $ipaddress = 'UNKNOWN';

    return $ipaddress;
}

// array of os's with their respective match in http_user_agent
$osList = array (
    /* -- WINDOWS -- */
    'Windows 10' => 'windows nt 10.0',
    'Windows 8.1' => 'windows nt 6.3',
    'Windows 8' => 'windows nt 6.2',
    'Windows 7' => 'windows nt 6.1',
    /* -- MAC OS X -- */
    'macOS High Sierra' => 'mac os x 10_13',
    'macOS Sierra' => 'mac os x 10_12',
    'Mac OS X El Capitan' => 'mac os x 10_11',
    'Mac OS X Yosemite' => 'mac os x 10_10',
    'Mac OS X Mavericks' => 'mac os x 10_9',
    'Mac OS X Mountain Lion' => 'mac os x 10_8',
    'Mac OS X (version unknown)' => 'mac os x',
    /* -- MOBILE -- */
    'Android Nougat' => 'android 7',
    'Android Marshmallow' => 'android 6',
    'Android Lollipop' => 'android 5',
    'Android KitKat' => 'android 4',
    'Android (version unknown)' => 'android',
    'iOS 11' => 'os 11',
    'iOS 10' => 'os 10',
    'iOS 9' => 'os 9',
    'iOS 8' => 'os 8',
    /* -- OTHERS -- */
    'Ubuntu' => 'ubuntu',
    'Linux (or Linux based)' => 'linux'
);

// function to get the client os
function get_os_name() {

    $user_agent = htmlspecialchars($_SERVER['HTTP_USER_AGENT']);
    $user_agent = strtolower($user_agent);

    global $osList;

    foreach($osList as $os=>$match)
        if (strpos($user_agent, $match))
            return $os;

    return 'Other';
}

// function to get the client browser
function get_browser_name() {
    $user_agent = htmlspecialchars($_SERVER['HTTP_USER_AGENT']);
    $user_agent = strtolower($user_agent);

    if (strpos($user_agent, 'opera') || strpos($user_agent, 'opr/'))
        return 'Opera';
    elseif (strpos($user_agent, 'edge'))
        return 'Edge';
    elseif (strpos($user_agent, 'chrome'))
        return 'Chrome';
    elseif (strpos($user_agent, 'safari'))
        return 'Safari';
    elseif (strpos($user_agent, 'firefox'))
        return 'Firefox';
    elseif (strpos($user_agent, 'msie') || strpos($user_agent, 'trident/7'))
        return 'Internet Explorer';

    return 'Other';
}

With respect to the OS and Browser functions; they're by no means catch-all. If you want a bulletproof (or more detailed) solution, you should look into a user agent parser. This should, however, be able to provide you with some useful information about 99% of your visitors.