If you move that declaration to a constructor promotion... it blows up. With an unclear error: "Bad and hasABus define the same property ($magic) in the composition of Bad. However, the definition differs and is considered incompatible." This is because the constructor promotion is just syntactic sugar, and the code the sugar is transformed into discards the default value in the declaration. (It becomes a line in the ctor instead.) https://3v4l.org/d1kV4#v8.3.6#php 2/2