For starting out I'd definitely recommend CodeIgniter (http://codeigniter.com) simply because the documentation is second to none.

Since learning CodeIgniter I've tried CakePHP and Zend, and found CodeIgniter is easier to use, and just as powerful, so I've stuck with it. In fact the BugMuncher (http://bugmuncher.com) back end & API is built on my custom version of CodeIgniter

Also check out the datamapper ORM plugin (http://datamapper.wanwizard.eu/) if you want Rails-like Active Record goodness.