Sorry for the confusing title....
We are developing an application to be used by multiple companies. For the most part, the application is the same, your standard sort of database manipulation pages (search pages, edit pages, etc.) customized for the data that it is designed for.
However, each company has a slightly different process, and we will be dealing directly with each company so we'd like to use some sort of system that would allow us to tweak pages depending on which company is viewing the page. For example, one company might want a couple extra fields on a data input page, or another company might want to view a different piece of data on a search results screen, and so on.
I understand this is all hypothetical and I wish I had a concrete example to give you, but honestly the companies haven't even given us very good examples. We just want to be ready.
So my basic question is, what is the most flexible way to allow for these tweaks and customizations on a per-company basis? Obviously, the most flexible but least programmer-friendly way would be to make a complete copy of the app for each company. This obviously isn't an option because we'd need to manage updating code on all the sites, trying to keep them all running and tested and having issues resulting from the customized code.
What are your thoughts on Smarty being a solution to this? Perhaps if we have a master set of templates, but then each company can have a different subfolder with any replacement template files... Of course we'd still need to update a bunch of different template files whenever we change one of them, but it would be a little more localized anyway.
Is there a better way? Some sort of differencing template engine maybe, so that we can still edit the original files and the changes will adapt on top of the originals (kind of like a patch)? Or perhaps we should use the object-oriented features of PHP5 and then use polymorphism? What is your best suggestion, and especially if you've had experience with this sort of thing, what are the options and which have you used and why?