RFC: https://wiki.php.net/rfc/enumerations

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>

Closes GH-6489.

Closes GH-6706

When a method is inherited, the static variables will now always
use the initial values, rather than the values at the time of
inheritance. As such, behavior no longer depends on whether
inheritance happens before or after a method has been called.

This is implemented by always keeping static_variables as the
original values, and static_variables_ptr as the modified copy.

Closes GH-6705.

Related to GH-6701

These are read-only properties, and Reflection makes sure to assign
only strings.

Closes GH-6692

This deprecates passing null to non-nullable scale arguments of
internal functions, with the eventual goal of making the behavior
consistent with userland functions, where null is never accepted
for non-nullable arguments.

This change is expected to cause quite a lot of fallout. In most
cases, calling code should be adjusted to avoid passing null. In
some cases, PHP should be adjusted to make some function arguments
nullable. I have already fixed a number of functions before landing
this, but feel free to file a bug if you encounter a function that
doesn't accept null, but probably should. (The rule of thumb for
this to be applicable is that the function must have special behavior
for 0 or "", which is distinct from the natural behavior of the
parameter.)

RFC: https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

Closes GH-6475.