I'd guess something that would run upon a LAMP stack would be the easiest to set up. In that case, I guess there would be some good choices in CodeIgniter and Zend Framework. Then there's Symphony and CakePHP, but I've heard mixed reviews (concerning stagnating community and/or tricky configuration) of the two latter.
Unfortunately, I haven't had the time to test thoroughly any of the four, because I've been busy coding my own framework, which I will not shamelessly (or otherwise) plug.
