How PHP determines whether it's a mobile or a computer visit

Ask the site to determine whether it is a mobile or computer access, and then display a different message

<?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, $s    
            UBSTR)) {return true;    
    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-', ' 160x160 ', ' 176x220 ', ' 240x240 ', ' 240x320 ', ' 320x The ', ' 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 ' Mobile login m.php.com ';  
else echo ' computer login www.php.com '; ?>