Mobile site/theme and url rewrite

Hello everybody!

I’m a old WP user/developer, but this is my first post here. I’m sorry for my bad english.

I want to create a mobile site, without using 3rd part plugin. I would like to passing a parameter to my index.php (like mobile=true) and loading a different theme.

index.php -> default_theme
index.php?mobile=true -> another_theme loaded via php

It’s possible?

Then I’d like to url rewrite:

/m/{query_string}/ -> /{query_string}/?mobile=true

How I can do it? (My wordpress is already installed in a subfolder, then my htaccess already exists)

Thank you very much,

Marco