How does php determine mobile client access?

// check if wap
function check_wap(){
 if(stristr($_SERVER['HTTP_VIA'],"wap")){// First check if it is a wap proxy, accurate High degree
 return true;
 }elseif(strpos(strtoupper($_SERVER['HTTP_ACCEPT']),"VND.WAP.WML") > 0){// Check whether the browser accepts WML.
 return true;
 }elseif(preg_match('/(blackberry|configuration/cldc|hp |hp-|htc |htc_|htc-|iemobile|kindle|midp|mmp|motorola|mobile|nokia|opera mini|opera |Googlebot-Mobile|YahooSeeker /M1A1-R2D2|android|iphone|ipod|mobi|palm|palmos|pocket|portalmmm|ppc;|smartphone|sonyericsson|sqh|spv|symbian|treo|up.browser|up.link|vodafone|windows ce|xda |xda_)/i', $_SERVER['HTTP_USER_AGENT'])){//Check USER_AGENT
 return true;
 }else{
 return false;
 }
}