Backend shared between sites - best method

I have a php backend which is to be used by multiple sites on a server I control. Now rather than having an individual co开发者_如何学Gopy in each web root, what is the best method to have each site work from 1 shared backend? (For easy updating of the backend, etc)

The site and backend are connected through the database. It doesn't matter where the backend and all your sites are located, as long as they all connect to the same database, they share the same information.

keep one copy of the core files that are common across all the sites. have all the sites link up to the core files. the non-core files can reside in its own sites folder. \