Ruby is more Object Oriented than PHP will ever be. You dont need interfaces or abstract classes in a dynamic language. Unless you want to feel "safe" as a developper.
http://metabates.com/2011/02/07/building-interfaces-and-abst...
http://docs.python.org/2/library/abc.html
Learn some OOP and design patterns before filling your mouth talking about PHP.
Also, Clojure is a dynamic language too and yet it craves for explicit types:
http://www.indiegogo.com/projects/typed-clojure
But this is only for noobs who "want to feel safe", according to you. What a joke. You probably can't even get past an introductory tutorial of this subject.
There is a reason why modern web development was invented with ruby and why all modern web frameworks try to copy rails or sinatra. Because Ruby was designed the right way.
One doesnt need interfaces and abstract classes if one uses duck typing and message passing the right way.
If you stayed a bit up to date, you would realize that Rails is pretty dated already. For instance, take a look at Symfony2. It easily beats Rails in every aspect possible. You probably won't be able to appreciate it though, since you don't know the first thing about design patterns. You just use whatever you thought was "cool", easy, and you stayed for the syntactic sugar, which you confuse with being well designed.