Stephan Schmidt has posted some thoughts about type hinting in PHP5 (what's there) and the same kind of functionality for objects in PHP6 (what's to come).

I was very pleased, that PHP 5 introduced type hints, although they are not available for primitives like string, int, boolean, etc. Still, I'd like to see the object type hint introduced in any future version of PHP that allows me to specify, that a method or function only accepts an object, regardless of the type of the object.

He gives the example of Java's handling of the functionality and what PHP does with the same type of code. He also mentions that, via the Reflection API extension, some of this is possible already, so why not just include it from here on out?