I'm starting to architect a quite complex web application. The implementation is probably going to be done in PHP, though if there are impressive reasons to choose a different environment I might be convinced.

I've looked at tools like Symfony and CakePHP. The problem is that it feels like they're relatively low level for a modern Web 2.0 application. They handle the basic things like MVC and scaffolding, but not the more advanced UI elements that I'm looking for. Here are some of my requirements:

* Single page architecture. With minor exceptions, there should be no page refresh. All actions are done via ajax, the way it's done in gmail, and to a lesser extent in Facebook.
* Ajax layout and widget handling. Not only the application doesn't refresh the page, but the developer can specify the layout and load various widgets into different parts of the page. This is somewhat like iGoogle, but should be better integrated.
* Support both on the client side and server side for AJAX widgets. It should be trivial to display the result of a select statement in an AJAX table/array like <http://developer.yahoo.com/yui/datatable/>. This should also apply to other widgets including
	+ Trees
	+ Menus
	+ Forms
* Speaking of forms, there should be easy integration with *client side* validation
* Signup/Authentication/Authorization. Including all the housekeeping things like forgot my password, CAPTCHAs, etc.

There's more, but I think I've given enough details so that you get an idea for what I'm looking for. Basically, I'd like to engineer a modern Web 2.0 app and skip writing, testing, debugging things that most web applications need to do. And yes, I know I can take YUI or jQuery and slap it on top of one of the regular platforms, but then I'd have to write all the glue. Now if there are modules that do this, that would be interesting. 
So if you say, take Symfony + modules xyz + jQuery and there's your answer, I'd be happy to hear that.

Finally, in terms of priority, I'm looking for something that's scalable, reliable, well engineered more than something that's easy to learn and deploy.