PHP 5 and Ruby are almost at opposite ends of the OO spectrum. PHP has some handy stuff like autoloading of classes (and it actually *has* classes). There the similarity ends.
Ruby lets you declare a class and then redeclare it. You can include a class file then open it up and completely overwrite (or rename) certain methods. You can also construct modules that are like classes but can be included into other classes giving their methods to the including class.
This may seem trivial but it allows for most of the power we see in Rails.
Also, there's the difference that all variables are actually instances of classes with methods that can be called. There's no such thing as "sometext"=>to_i() in PHP.