Re: [RFC] Readonly Variables

Hi Ben, > On 24. Feb 2026, at 01:45, Ben Ramsey <ramsey@php.net> wrote: > > According to this RFC, both of these would be forbidden: > > readonly $a = new Foo(); > $b = $a; As Tim already clarified, there is a common misconception about how PHP handles objects internally. In PHP, objects are not passed by reference in the sense of the & operator, but via an object identifier. This means that $b = $a simply copies the object identifier, not a reference to the variable itself, and is therefore perfectly valid: ``` <?php readonly $a = new Foo(); $b = $a; ``` The readonly flag only prevents re-assignment of $a itself. The & operator, on the other hand, would create a true reference to the variable $a, which would allow $a to be modified indirectly through $b, effectively circumventing the readonly flag. This is why taking a reference of a readonly variable is forbidden. I hope this clears up the concern. > and: > > function modify(Foo $value): void {} > readonly $a = new Foo(); > modify($a); > > I said "according to this RFC," but the RFC doesn't mention object behavior with > readonly at all, which I think might be an oversight. The only place it hints at allowing objects is > in the examples with new PDO(). > > Is the idea to allow objects (and arrays) similar to how JavaScript handles these with > const? That is, the objects and arrays themselves are mutable, but > the variable name cannot be reassigned to a new object or array? Regarding your question about object and array behavior: you are correct that the RFC did not explicitly document this, which was indeed an oversight on my part. As already mentioned above, the variable binding is immutable, meaning $a cannot be reassigned to a different object, but the internal state of the object can still be modified. This is also consistent with how the readonly attribute behaves for class properties: ``` <?php readonly $a = new Foo(); $a->value = "foo"; // Valid: modifying the object's internal state is allowed $a = new Foo(); // Error: Cannot re-assign readonly variable ``` For arrays, however, the behavior differs from JavaScript. In PHP, arrays are value types with copy-on-write semantics, not reference types. This means that a readonly array is fully immutable, as neither the array itself nor its elements can be modified after the initial assignment: ``` <?php readonly $arr = [1, 2, 3]; $arr[] = 4; // Error: Cannot re-assign readonly variable $arr[0] = 99; // Error: Cannot re-assign readonly variable ``` I have added explicit examples for both objects and arrays to the RFC to make this behavior clear. I appreciate you flagging this. (I apologize for the duplicate email, Ben. The first attempt was sent from an incorrect address and did not go through properly.) Cheers, Joshua Rüsweg