As written this feels too close to multiple inheritance. We already have traits. Why not allow an interface to reference a trait as the default implementation and then allow a class to optionally use that trait through the interface reference.
We spend a lot more time reading code than writing it. The elegance of short closure combined with the convenience of variable scope usage has already shown to be a game changer on Typescript and there doesn’t seem to be any technical issue with having it on PHP.