[zarakshR]

> because PHP and Javascript will implicitly convert back and forth

If you can provide (valid!) methods `T -> U` and `U -> T` for two types, why wouldn't `T = U` hold? (Atleast for types where `=` makes sense)

This is the definition I am using:

> S is a subtype of T, written S <: T, if a value of type S can safely be used in any context where a value of type T is expected.

from Pierce's "Software Foundations"

Of course, you may not want to make `String <: Number` and `Number <: String` (and thus `String = Number`), because there is no sensible way to do so; but this is an issue with the way PHP/JS handles subtyping, not with the notion of subtyping itself and certainly does not apply to the example of nullable types.

Unfortunately, I am not familiar enough with C++ to comment on the other question, sorry!