[eluser]BrianDHall[/eluser]
PHP4 vs PHP5 style OOP - CI is backwards compatible by avoiding php5-only functions in the core, or when they do use them they try to provide core functions to make it work even on php4 systems. But there is nothing that prevents php5 functionality at all. In fact most extensions to CI require php5, including many of the auth systems and most (if not all) ORM systems.
As to your project, I came to CI in nearly _exactly_ the same position as you. I had a project built by someone else that needed redone and fixing (it wasn't really done at all), had never used OOP at all, and was used to 1-man spaghetti code. CI allows you to improve while not requiring you to learn a 1000 things at once.
I think you'll find it wonderful to work in, and as you learn you may very well find that CakePHP is not particularly more powerful or faster to develop with - CI just does some things very differently than other frameworks (like no command line required), and it seems like it is 'missing' those features. Soon you'll find in CI that with a little proficiency you can largely eliminate CRUD (create, retrieve, update, delete database functionality), creating new controllers/models/views takes no more time than calling up a command line, and the lack of some magic/convention functions can often speed development by making it easier to wrap your head entirely around what is going on.
One tip - on my first project I skipped prebuilt auth systems and ORM to just develop FAST, because I needed a totally rebuilt system in a WEEK that the last developer had 3 months to work on. I couldn't spare the extra day or two it might take to get up and running with them.
On auth, I'm still undecided, as I still use my own built systems. On ORM I use Datamapper OverZealous Extention available over in ignited code forum - which I dearly love, but it may be best to start without it to not overtax your brain. It's very magical, as all ORM systems are, as much as I love it now I don't know that I'd have appreciated it when I started.
Anyway, my Really Import Tip #1 - get a debugger. Seriously, half the problems posted in this forum would be unnecessary if they had a debugger.
I found an IDE with PHP syntax checking really helped, especially with OOP. It's hard to get use to Object::function() and $this->object->etc and when working inside controllers and models there are those extra brackets of the class and function you are working in, its easy to make a mistake even once you are very experienced.
But having a debugger, jeeze. What a difference its made. I still sometimes use var_dump() for a quick peek at a variable, but when anything more complicated is needed stepping into debug mode and advancing through your code one line at a time provides remarkable insight. I personally use Netbeans with its built-in support for Netbeans, and then either WampServer or XAMPP for the localhost server/debugging environment.
The forums here have the info on getting them working with CI, and it's easy (one copy/paste into your php.ini file, maybe a download of a single file for xdebug, and a switch in your CI's config.php file) - and the plus is you learn to setup CI to support query strings, which I think it really should do by default anyhow.
Good luck on your project, and welcome to CI! I think you'll find this one of the more helpful communities around, and CI to be a shockingly easy framework to get into.
I'll warn you its easy to get so used to it to feel you want more challenge, like maybe there are better tools to help you work faster. Note that you don't have to go to a different framework for them, they are available over in Ignited Code or in the forum. Auth, HMVC, ORM, magic/convention (rails style) functionality, use of Zend Framework libraries (they have a lot of them), you name it - it's available in CI, you don't even have to learn another framework to use them. You just aren't required to use them to get started
