If you're especially looking to develop for Maemo on the N900 (and maybe some day for MeeGo), go for PyQt instead.

Qt is the GUI library (though that term might be misleading, since Qt does a lot more than just handle the GUI part of your applications) that will be best supported on both OS, so you might want to look into that one instead of other GUI solutions.

Qt is natively C++, but there's the aforementioned PyQt bindings that combine the strengths of Qt and the comfort of Python.

Python - despite the obvious syntax differences - should feel pretty natural for a PHP coder, so I'd say the Python + PyQt combination should suit your needs pretty well.

(Quick sidenote: Python + PyQt has some well-made classes for accessing websites, complete with full auth and cookies - so server-client based applications with a PHP server in the background are quite easy to make.)

There's not much work involved with getting your workspace up and running, either. If you're on Linux, must grab the Eric Python IDE and you'll have everything you need right at your fingertips, since it already includes god-sent tools like Qt Designer and Qt Linguist.

For a short intro to Python + PyQt, here's a nice little starter tutorial made by mikec.

I'm totally new to the mobile app development stage myself, but if there's anything you want to know I'll try to help.

Oh, and welcome to the Maemo development world...