How to architect an web and mobile application?

How to architect an web and mobile application?

I'm in a situation where I need to design an application that will have a
web application (to be developed initially), as well as separate iOS,
Android and Windows Mobile apps.
As such, I'm thinking of developing the bulk of the application as a
RESTful API (probably in PHP).
The way I see it is that the website will be served as another another PHP
web application which will handle all it's session and other website
specific stuff and communicate via REST to the main PHP REST application.
The mobile applications will also communicate via REST with the PHP REST
app.
I just wanted to get some opinions regarding laying things out in this
way, especially in regards to the web app. Does it make sense for the web
application to be served by a PHP app communicating with the API or would
you suggest that the web app bypass the RESTful API and just use the
business objects directly, leaving the RESTful API just for the mobile
applications?