What follows is my experience with Java, PHP and Ruby. I mainly use PHP as a dynamic language that supports duck typing but also the definition of Java-like interfaces, but does not force any of the two approaches as you can define interfaces whose method arguments accept any variable or not using interfaces at all. Is duck typing that a revolution?

He shares some of the common misconceptions he's seen including the idea that duck typing can help completely different objects work together and that, sometimes, despite naming conventions, functionality was intended to be different. He shows how even interfaces in PHP can be implemented loosely and the "acts as" and "single callback" architecture ideas.

Well, if you are into duck typing style of programming, it may be interesting for you to have an object that implements certain set of functions, but not necessary declares it at class definition. Languages like Smalltalk do it all day along, so why PHP couldn't?

His example defines an interface Cow and a class MooingGrassEater and a function, CowConsumer, that does the work. A classname is passed in and an instance of that class is checked with "implements" rather than "instanceof" to see if it uses the Cow interface. He points out a place where PHP itself uses something similar in user defined streams.