Eran Gelperin gives an overview of the current state of overloading abilities PHP has in a new blog post today:

Operator overloading is a programming language features that allows operators to act differently depending on the type of data they are operating on. Since OOP lets us create custom types (classes), there are plenty of opportunities to do useful and interesting code manipulations using operator overloading.

He talks about magic functions, the additions that the SPL made, the PECL addition operator and how much its currently being discussed on the PHP internals list.