php是如何判定手机、电脑访问自动跳转头文件的

<?php 
function is_mobile_request()  
{  
 $_SERVER['ALL_HTTP'] = isset($_SERVER['ALL_HTTP']) ? $_SERVER['ALL_HTTP'] : '';  
 $mobile_browser = '0';  
 if(preg_match('/(up.browser|up.link|mmp|symbian|smartphone|midp|wap|phone|iphone|ipad|ipod|android|xoom)/i', strtolower($_SERVER['HTTP_USER_AGENT'])))  
  $mobile_browser++;  
 if((isset($_SERVER['HTTP_ACCEPT'])) and (strpos(strtolower($_SERVER['HTTP_ACCEPT']),'application/vnd.wap.xhtml+xml') !== false))  
  $mobile_browser++;  
 if(isset($_SERVER['HTTP_X_WAP_PROFILE']))  
  $mobile_browser++;  
 if(isset($_SERVER['HTTP_PROFILE']))  
  $mobile_browser++;  
 $mobile_ua = strtolower(substr($_SERVER['HTTP_USER_AGENT'],0,4));  
 $mobile_agents = array(  
    'w3c ','acs-','alav','alca','amoi','audi','avan','benq','bird','blac',  
    'blaz','brew','cell','cldc','cmd-','dang','doco','eric','hipt','inno',  
    'ipaq','java','jigs','kddi','keji','leno','lg-c','lg-d','lg-g','lge-',  
    'maui','maxo','midp','mits','mmef','mobi','mot-','moto','mwbp','nec-',  
    'newt','noki','oper','palm','pana','pant','phil','play','port','prox',  
    'qwap','sage','sams','sany','sch-','sec-','send','seri','sgh-','shar',  
    'sie-','siem','smal','smar','sony','sph-','symb','t-mo','teli','tim-',  
    'tosh','tsm-','upg1','upsi','vk-v','voda','wap-','wapa','wapi','wapp',  
    'wapr','webc','winw','winw','xda','xda-'
    );  
 if(in_array($mobile_ua, $mobile_agents))  
  $mobile_browser++;  
 if(strpos(strtolower($_SERVER['ALL_HTTP']), 'operamini') !== false)  
  $mobile_browser++;  
 // Pre-final check to reset everything if the user is on Windows  
 if(strpos(strtolower($_SERVER['HTTP_USER_AGENT']), 'windows') !== false)  
  $mobile_browser=0;  
 // But WP7 is also Windows, with a slightly different characteristic  
 if(strpos(strtolower($_SERVER['HTTP_USER_AGENT']), 'windows phone') !== false)  
  $mobile_browser++;  
 if($mobile_browser>0)  
  return true;  
 else
  return false;
}
    
if(is_mobile_request()){ 
header("location:app/index.html");
exit();
}
else{ 
header("location:index/index.html");
exit(); 
}
?>