How can a PHP program reasonably structure the PC and mobile websites to automatically switch access?

The programs are the same set, and the domain name structure is as follows:
PC domain name: www.xxxx.com Mobile domain name: m.xxxx.com
How to reasonably build automatic switching access between PC and mobile phones,
Don’t tell me about my mobile phone Add a function judgment when accessing the terminal,
In addition to different templates, how can the terminal control it more reasonably

Reply content:

The programs are the same, and the domain name structure is as follows:
PC domain name: www.xxxx.com Mobile domain name: m.xxxx.com
How to reasonably build automatic switching access between PC and mobile phones,
Don’t tell me about my mobile phone Add a function judgment when accessing the terminal,
In addition to different templates, how can the terminal control it more reasonably

The server can be exactly the same. . . Just distinguish the front-end template in the function that renders the page

Responsive layout?