Equivalent of Django/Turbogears for PHP?

Hi

I'm trying to find out whether to use Python or PHP to write web apps.
Since PHP is available on just about every web hoster while Python is
much more rare, I'd like to check if some equivalent servlet engine to
Django or Turbogears exist for PHP, and, obviously, that they are
usually available with PHP on shared servers.

And I'll take advantage of this thread to ask what the advantage is in
using a servlet engin over writing web apps the standard way as seen
in books, ie. PHP code in pages that are fetched and executed by the
interpreter when called?

Thank you.