Codeigniter is an amazing implementation of a framework but i find it to be less scalable than I would wish on the type of projects I am working on.
I am actually looking into the zend framework because if anyone knows how to build a framework on php they do. The problem I am finding is that there is no reliable information on how to structure systems (basic application development) aimed at being reliable, secure, stable and performance.
I have been delving into smarty template engine over the past few months and find it to be substantial and defiantly a library that I would use in future projects.
I'm supprised at the lack of books available for online application development all php related books seem to be aimed at the basic php language and a few libraries and at most have a couple of paragraphs on the MVC.