I can't comment on Solar or Yii because I've never used them. When I've given both a cursory look, they did not seem as stable as frameworks I've used before. I've developed with CakePHP for years and have run into the PHP 4 barrier with it, where continuing to develop for PHP 4 does not make sense any longer. Ultimately, it just costs more money.
If you are looking for a framework to build upon, I would recommend looking into the following:
All are very modular by nature (including Solar and Yii), with support for integrating excellent tools like the Doctrine ORM. Some assume Doctrine is your ORM of choice. Honestly, it should be.
ZF is decidedly PHP5 only, which I like. I also appreciate the modularity of it. I can create a base "project" and then add individual "apps" to it, each with their own models/views/controllers. You can also use its classes/features individually without the entire framework bootstrapping process. It is backed by Zend, and is very mature -- I'm assuming because so many trained eyes have used and improved it. The consequences of this are fantastic: just browse through the web site, follow the quick start and look at the available components in the reference guide and you'll see what I mean.
Lithium was forked off from CakePHP's Cake3 project, aiming for a framework with support only for PHP 5.3 and PHP 6. It is decidedly PHP 5.3 only, which means it has hitched its wagon to the newest stable PHP with the most features, such as anonymous functions, a native MySQL driver, and much more. Worth looking into if you are able to move your team to a PHP 5.3-only environment, especially if you have people who know the value of more modern programming techniques found in languages like Python or Ruby. It is a young framework, but has much of the same talent behind it that CakePHP has had, so that gives it a huge thumbs up in my book.