How do I use mod_rewite to use a cached copy of a page if available otherwise run a dynamic script?

I would like to save the generated output of php scripts into a folder structure that apache checks first.

/about/index.php <- dynamic should be rendered only if cache below does not exist/contact/index.php/public/about/index.html  <- cached should be rendered first if exists/public/contact/index.html

How do I create an RedirectCond/RedirectRule set to do this for me?