Where to put certain code

[eluser]jaswinder_rana[/eluser]
Hi:

I have just downloaded CodeIgniter. I am not sure about where to place certain code.

I have started my own application. My first part is to build authentication system. I have the tables and logic planned out. I have already done many authentication system but I wanted to do it with CodeIgniter this time.

I have few questions:

- Where do I place code for login
- Where do I place code for checking login for users


Some different topics are:
- If I have a header view, do I need to include it in every Controller? Or is there a way to automatically include it in every controller?
- If I have sidebars, and some content might be page specific and some application wide. What is the best way to deal with them? Call Models in every controller?


As you can see, it's overwhelmed me a little.

I know (and am reading) CodeIgniter manual. But, are there some sample applications, say a sample blog system or something, that I can look at and see how they do things?

Oh and totally off topic question, is there plan to write PHP5 only CodeIgniter?

Please help.

Thanks