这些类型声明只能在类中使用。
The value must be an instanceof the same class as the one
in which the type declaration is used.
值必须是 instanceof 使用了类型声明的父级类。
static is a return-only type which requires that the
value returned must be an instanceof the same class as the one
the method is called in.
自 PHP 8.0.0 起可用。