Hi,
As of today, readonly properties cannot have a default value:
class UltimateQuestion {
readonly int $answer = 42; // Fatal error: Readonly property Question::$answer cannot have default value
}
The rationale given in the original RFC (https://wiki.php.net/rfc/readonly_properties_v2) was:
As the default value counts as an initializing assignment, a readonly property with a default value is essentially the same as a constant, and thus not particularly useful. The notion could become more useful in the future, if new expressions are allowed as property default values. At the same time, depending on how exactly property initialization would work in that case, having a default value on a readonly property could preclude userland serialization libraries from working, as they would not be able to replace the default-constructed object. Whether or not this is a concern depends on whether the property is initialized at time of object creation, or as an implicit part of the constructor (or similar). As these are open questions, the conservative choice is to forbid default values until these questions are resolved.
A new fact is that properties will be able to be declared in interface since PHP 8.4 (as introduced in the recently accepted property hooks RFC). It is true that, for an individual class, a readonly property is functionally equivalent to a constant. But my class implements an interface (or, before 8.4, follows a non-written protocol), and whether that particular implementation choose to always hold the same value for a specific property is irrelevant.
interface Question {
public int $answer { get; }
}
class UltimateQuestion implements Question {
readonly int $answer = 42;
}
Therefore I think it is reasonable for readonly properties to accept default values.
About the concern given in the original RFC: “having a default value on a readonly property could preclude userland serialization libraries from working, as they would not be able to replace the default-constructed object”… TBH, as I almost never serialise my objects, and even less often customise serialisation, I can only guess what the effective issue is. My best guess is that a custom __unserialize() method would not be able to reinstate the incriminated readonly properties, because those would already have been initialised with the default value. If this is the case, a solution is to do for __uninitialize() the same thing that we have done for __clone(), see: https://wiki.php.net/rfc/readonly_amendments#proposal_2readonly_properties_can_be_reinitialized_during_cloning
—Claude