Nikita Popov has posted the second part of a series looking at how PHP 7 represents values internally. In the first part of the series the focus was on the major change from PHP 5: the zval updates and how they're allocated. This new post gets into more of the details on each of the types and how they're handled.
In the first part of this article, high level changes in the internal value representation between PHP 5 and PHP 7 were discussed. As a reminder, the main difference was that zvals are no longer individually allocated and don’t store a reference count themselves. Simple values like integers or floats can be stored directly in a zval, while complex values are represented using a pointer to a separate structure.
[...] In the following the details of the individual complex types will be discussed and compared to the previous implementation in PHP 5. One of the complex types are references, which were already covered in the previous part. Another type that will not be covered here are resources, because I don’t consider them to be interesting.
He goes through a few of the different types including strings and arrays and then gets into detail on how objects have changed from PHP 5 to PHP7. He also talks about "indirect zvals" (the IS_INDIRECT handling) that points to another zval instance rather than embedding it. Finally, he talks about two other constants, IS_CONSTANT and IN_CONSTANT_AST, and how they're used behind the scenes with some example code to illustrate.