How can a set of php programs reasonably enable automatic access to websites on PC and mobile phones?

The program is the same. the domain name structure is as follows: PC-side domain name: www.xxxx.com mobile-side domain name: m.xxxx.com how to reasonably build a PC-side and mobile-side automatic access switch, don't tell me how to add a function to judge when accessing the mobile phone. in addition to different templates, how can the program end be more reasonable... the program is the same, and the domain name structure is as follows:
PC domain: www.xxxx.com mobile domain: m.xxxx.com
How to reasonably build automatic access between the PC and the mobile phone,
Do not tell me to add a function when accessing the mobile phone,
In addition to different templates, how can the program end perform more reasonable control?

Reply content:

The program is the same, and the domain name structure is as follows:
PC domain: www.xxxx.com mobile domain: m.xxxx.com
How to reasonably build automatic access between the PC and the mobile phone,
Do not tell me to add a function when accessing the mobile phone,
In addition to different templates, how can the program end perform more reasonable control?

The server can be exactly the same... You can use different front-end templates to render page functions.

Responsive layout?