PHP has some OO like capabilities, but mostly it is procedural based.
You can write your code to be OO like, and with PHP5, it's going to have a lot more OO style functions and tools.
PHP Nuke, XOOP, and POST-Nuke are all Content Management Systems (CMS) as some would refer to them. They provide a portal-like way to deliver information to users and allow that content to be customizable.
I have heard great reviews for all of them although I have never used any of them. I prefer to write all of my own solutions from scratch.
That said, I wouldn't really be a good person to ask if one or another is appropriate for a company intranet.