A relatively mature and easy-to-develop forum system with a mobile interface

It is easier to develop secondary, this is more important

Reply content:

It is easier to develop secondary, this is more important

PHP:
https://phphub.org/
http://hifone.com/
http://www.startbbs.com/

ruby:
https://ruby-china.org/

node:
https://cnodejs.org/

rust:
http://rust-lang-cn.org/

I believe you can find the source code address from several websites

Do you need a traditional forum or a light forum?
If it is a traditional forum, such as Discuz! and PHPWind, they are too bloated and complex, and it is not easy to develop them twice.
If it is a light forum, the better ones are Carbon Forum, HYBBS, etc. The disadvantage is that they do not have as many functions as traditional forums.
If you don’t need extra features, I recommend you check out these light forum websites:
Carbon Forum (strongly recommended, available in Android and iOS versions)
HYBBS (developed based on the streamlined framework of ThinkPHP, Upgrade of discontinued functionality)
RocBoss
HadSky
I saw Laravel in your tags, so I added this: Flarum