In all of the current systems, I would avoid an object oriented design except where the situtation clearly calls for one.
PHP4 supports OO programming, but is admittedly weak at enforcing it. For example there are no truly private methods or properties, but with some discipline you can write as though there were.

We've been doing this for several months. (Before that it was used by some developers but not all, and not consistently when it was used.) We recently installed PHP5 on one server to see how much porting effort would be involved in migrating all of our sites. So far everything that has been written in the past several months, and much of what came before that, ran without modification. The advantage is that PHP5 will allow us to enforce the best practices that are currently only enforced through policy.