I am writing a web application using PHP. I want to use the MVC pattern for this, and decided to go with CodeIgniter. My application will have some pages which will require authentication, and some pages won't. I want to design this in a very generic way, so that there should be no code duplication. Can any one point to some good "design/class structure" for this?
I was looking into the same thing recently, and I found a CodeIgniter fork called Kohana that includes a nice authentication module. If you are set on CI, maybe adapting Kohana's auth module backwards to CI would save you some time? If you have just started out on your project and PHP5 is OK to use, consider switching over; they are very similar frameworks.
May be you can use CL_AUTH library for CI. I've used it and it works good. You can find it here http://www.jasonashdown.co.uk/cl_auth_doc/