Julien Pauli has a recent post to his site that "zooms in" on objects and classes with a look behind the scenes at how they're handled in the PHP source (at the C level) with plenty of code examples and explanations as to how they work.

Everybody uses objects nowadays. Something that was not that easy to bet on when PHP5 got released 10 years ago (2005). I still remember this day, I wasn't involved in internals code yet, so I didn't know much things about how all this big machine could work. But I had to note at this time, when using this new release of the language, that jumps had been made compared to old PHP4. The major point advanced for PHP5 adoption was : "it has a new very powerful object model". That wasn't lies. [...] Here, I will show you as usual how all this stuff works internally. The goal is always the same : you understand and master what happens in the low level, to make a better usage of the language everyday.

The article does a great (if lengthy) job of covering everything that happens with PHP's objects and class system, including stats about memory consumption. He includes both the PHP code and the C code to illustrate what's happening with classes, interfaces, traits and object methods/attributes (including object references). He also talks about what "$this" is and how class destructors are handled.