[eluser]Myles Wakeham[/eluser]
If you are going to develop more than small scripts in PHP, you'll want to embrace OOP. The fact that you are using CI suggests you are trying to do something other than simple scripts in PHP. Like most programming languages these days, PHP and CI embrace OOP and kinda assume that you are going to use it if you are doing anything serious.
Also CI is based around the MVC design pattern, which is closely coupled with OOP.
In a perfect world, you'd learn OOP, then MVC design pattern fundamentals, then PHP5, etc. If you are doing this in a different order, try and focus on the missing parts of your study so that all of these items are given the appropriate attention and you'll be happy you did.