参考

https://github.com/serbanghita/Mobile-Detect

安装

composer require mobiledetect/mobiledetectlib

PC 端/平板网站

域名：www.domain.com

require 'vendor/autoload.php'; 

$detect = new Mobile_Detect;

if ($detect->isMobile()) {
  header('Location: http://m.domain.com/');
}

移动端网站

域名：m.domain.com

require 'vendor/autoload.php';

$detect = new Mobile_Detect;

if (!$detect->isMobile()) {
  header('Location: http://www.domain.com/');
}