How does PHP determine the type of user device accessing the system (code example)

/**
 * 用户设备类型
 * @return string
 */
function clientOS() {

    $agent = strtolower($_SERVER['HTTP_USER_AGENT']);

    if(strpos($agent, 'windows nt')) {
        $platform = 'windows';
    } elseif(strpos($agent, 'macintosh')) {
        $platform = 'mac';
    } elseif(strpos($agent, 'ipod')) {
        $platform = 'ipod';
    } elseif(strpos($agent, 'ipad')) {
        $platform = 'ipad';
    } elseif(strpos($agent, 'iphone')) {
        $platform = 'iphone';
    } elseif (strpos($agent, 'android')) {
        $platform = 'android';
    } elseif(strpos($agent, 'unix')) {
        $platform = 'unix';
    } elseif(strpos($agent, 'linux')) {
        $platform = 'linux';
    } else {
        $platform = 'other';
    }

    return $platform;
}