In a new article on his blog Frank De Jonge demonstrates a lesser known feature/bug of PHP.

When two objects are of the same type, they can access the private properties of the other. This might seem like a bug, but it's actually a very valuable feature. For instance, it allows you to perform very efficient equality checks without the need to publicly expose your internals.

http://blog.frankdejonge.nl/pipelines-immutability-and-privates/