Numeric array indexes now convert to string property names when an array is cast as an object.
The reverse direction is similar; numeric string property names now map back to integer array keys.
Previously, these transformations would give you inaccessible indexes or properties.
References to undefined constants (i.e. strings without quotes around them) now generate warnings instead of notices, though they still work for now.
This will turn into an Error eventually (<= PHP 8.0).