My suggestion would be to initially broaden your horizons when attempting to understand OOP and what it is all about. Look back into more developed arenas such as Java. Or (and), go more theory based by looking at Object Orientation in a broader sense -- it's relation to other programming paradigms (there are lots). There is no shortage of valuable information available either digitally or in (often huge tomes) books.

If you restricted your learning specifically to PHP's OOP implementation then, in my own point of view, you wouldn't be getting the best out of your time. Nor would you get the whole picture, or anything close.

Are there any areas that you've looked into so far and had trouble understanding (please don't say, "the whole concept" )?