#[PostMeta]
Attribute on classes.
Declares a class of post meta fields: each promoted constructor parameter is a
field, stored in its own meta key {namespace}_{field} (field name in snake
case). The fields are reached through the post object only:
$event->meta(EventDetails::class).
The class is not readonly: the fields are usually private, with getters and
setters, and save() of the post writes the changed values.