Don't overload ArrayObject get_properties

[nikic]

Change to fix#3573 (comment).

The first commit lays groundwork to allow proper overloading of (array) casts. The second commit explicitly overloads (array) for ArrayObject and removes the get_properties overload.

The BC breaks are noted in UPGRADING. I think that array_key_exists() is the only problematic one. However, as we already plan to deprecate array_key_exists with objects. As this will make the operation generate a warning (rather than silently change behavior) I'm not so concerned.

[nikic]

Overall I'm not really happy with this change, as it has potential impact on too many places.

I think a better solution might be to introduce a get_properties_for handler, which accepts a purpose flag. We also need this to solve long standing bugs in DateTime and other classes that want to overload get_properties only for some specific functionality, but end up overloading it for everything.

[dstogov]

This is definitely not a simple bug fix :(
I'll try to think about a better solution.

[dstogov]

@nikic SPL is over-designed, and provides more functionality that actually can do :(
Ideally, I would drop things that are not implemented properly and can't be fixed.

If you decide to proceed with get_properties_for(), it may make sense to just change prototype of get_properties(). Object handlers are going to be broken by typed properties, anyway.

[nikic]

Closing in favor of #3579, which has landed.