separate different version of a website

I want to make sure I want to optimize and make my site very easy to maintain for different version of my site.

I have few version of my site:

• iphone/ipod/android etc...
  


• ipad/tablets etc...
  


• other small devices like old clap phone
  


• default
  

I use ubuntu server with MySQL 5, PHP 5 and Apache + Memcache.

What would be the best way to implement my site so they all use the same base functionality:

• PHP
  


• JS (for common)
  


• CSS (for common)
  


• etc...?
  

thanks

Source: Tips4all