Looking for php files in two places and overriding the file if exists in the main directory

Looking for php files in two places and overriding the file if exists in
the main directory

I would like to have some base files to be used across two sites, and then
have them override the base file if there is a file found in lets call it
the parent/main directory. Otherwise look in the secondary base directory.
I am developing a site that is pretty much a duplication, but there are
only a few files that would be different. So in order to save me copying
and pasting the whole site a feature like this would be very useful.
Has anybody come across a similar type of requirement, any information
would be help.
I am also using Smarty Templating here.
Thanks