How does php determine whether the client is a pc or a mobile phone?

How can I determine whether a web page is viewed on a pc or a mobile phone? How can I determine whether a web page is viewed on a pc or a mobile phone?

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How can I determine whether a web page is viewed on a pc or a mobile phone?

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

  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';elseecho 'pc';?>

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Judge UA.