why wouldn't ['foo'] be a JSON array structure ?

This method handles an edge case: when a json like [{"0": 5}] is decoded to PHP, accessing @[0] on the inner object should return nothing, because numeric index selectors only work on arrays, not objects per the RFC. So just calling array_is_list() would incorrectly allow it. The || part of the condition specifically excludes single-element arrays with index 0 set to handle this distinction.

Does this answer your question?

but this ignores 2 cases:

• AFAICT, it breaks the case of accessing single-element arrays
• you could totally have JSON objects with multiple numeric keys ({"0": 5, "1": 12}), which would be considered array structure in your method.

It seems pretty complex to fix... Just in case, @mtarld, is there something in JsonStreamer that could also help here? We would need to "easily" differentiate an array from an object with numeric indices. Maybe Read/Lexer.php?

For the case of strings using json_decode, we might decode objects into stdClass to distinguish objects from arrays. But this suffers from other limitations as this won't support object keys that are not valid names of PHP properties.

Tried this approach in 54281cb and succeeded on making the CI green. It makes things a bit more complicated but we can easily differentiate objects and arrays now. Also updated the compliance test suite because the specific cases we were dealing with suffered from the same problem when being decoded from cts.json.

this suffers from other limitations as this won't support object keys that are not valid names of PHP properties

Do you have an example? It seems to work pretty well, and as far as I know, stdClass are actually "allowed" to use wrong property names like https://3v4l.org/ede7M#vnull.