How does PHP determine whether the client is a PC or a mobile phone?

How to determine whether you are browsing a web page on a PC or a mobile phone?

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How to determine whether you are browsing a web page on a PC or a mobile phone?

PHP There is a very famous mobile phone detection library Mobile-Detect
Address: https://packagist.org/packages/mobiledetect/mobiledetectlib

<code><?php
function isMobile(){
    $useragent=isset($_SERVER['HTTP_USER_AGENT']) ? $_SERVER['HTTP_USER_AGENT'] : '';
    $useragent_commentsblock=preg_match('|\(.*?\)|',$useragent,$matches)>0?$matches[0]:'';
    function CheckSubstrs($substrs,$text){
        foreach($substrs as $substr){
            if(false!==strpos($text,$substr)){
                return true;
            }else{
                return false;
            }
        }
    }
    $mobile_os_list=array('Google Wireless Transcoder','Windows CE','WindowsCE','Symbian','Android','armv6l','armv5','Mobile','CentOS','mowser','AvantGo','Opera Mobi','J2ME/MIDP','Smartphone','Go.Web','Palm','iPAQ');
    $mobile_token_list=array('Profile/MIDP','Configuration/CLDC-','160×160','176×220','240×240','240×320','320×240','UP.Browser','UP.Link','SymbianOS','PalmOS','PocketPC','SonyEricsson','Nokia','BlackBerry','Vodafone','BenQ','Novarra-Vision','Iris','NetFront','HTC_','Xda_','SAMSUNG-SGH','Wapaka','DoCoMo','iPhone','iPod');
     
    $found_mobile=CheckSubstrs($mobile_os_list,$useragent_commentsblock) || CheckSubstrs($mobile_token_list,$useragent);
     
    if ($found_mobile){
        return true;
    }else{  
        return false;
    }
}
if (isMobile())
echo 'phone';
else
echo 'pc';
?></code>

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Just judge UA.