How does PHP determine computer access or phone access?

The number of mobile internet users is increasing, now the website has launched a mobile website, computer users access to the computer version of the Web page, when users visit the website through the mobile phone automatically jump to the mobile version of the page, Below to share a section of PHP to determine the computer access or mobile Access code:

<?php//Mobile Web page Jump//If the browser is detected as one of the following specified mobile browsers, return truefunction Is_mobile () {$regex _match= "/(nokia|iphone|android| Motorola|^mot\-|softbank|foma|docomo|kddi|up\.browser|up\.link| "; $regex _match.= "htc|dopod|blazer|netfront|helio|hosin|huawei|novarra| Coolpad|webos|techfaith|palmsource| "; $regex _match.= "blackberry|alcatel|amoi|ktouch|nexian|samsung|^sam\-|s[cg]h|^lge|ericsson|philips|sagem|wellcom   |bunjalloo|maui| "; $regex _match.= "Symbian|smartphone|midp|wap|phone|windows Ce|iemobile|^spice|^bird|^zte\-|longcos|pantech|gionee |^sie\-|portalmmm| "; $regex _match.= "jig\s browser|hiptop|^ucweb|^benq|haier|^lct|opera\s*mobi|opera\*mini|320x320|240x320|176x220";       Regex_match.= ")/I"; return isset ($_server[' http_x_wap_profile ')) or isset ($_server[' http_profile ']) or Preg_match ($regex _match, Strtolower ($_server[' http_user_agent '));} $is _mobile=is_mobile ();  if ($is _mobile) {//This is a mobile browser, you can jump to the mobile version of the page//header ("location:http://www.abc.com/3g"); echo "Mobile Access"; }else{//This is not a mobile browser//header ("location:http://www.abc.cOm/desktop "); echo" Computer access "; }?>

How does PHP determine computer access or phone access?