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Date: Wed, 29 Mar 2023 21:53:56 +0200
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Subject: [PHP-DEV] [RFC] Properties in interfaces
From: fekzol.13ker@gmail.com (=?UTF-8?Q?Zolt=C3=A1n_Fekete?=)

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=EF=BB=BFHi,

[Proposal]
Allow interfaces to define properties. The implementing class must
have the same properties with  the same name/type.

As the goal of the interfaces is to define the public "surface" of
the implementing class, I think that the public properties are also
part of that.


[Use case] - tl;dr;
Right now I=E2=80=99m working on a product configurator. The input I get, I
turn them into ValueObjects in the initial phase of the request. Then
later based on some conditions, I turn them in a deterministic but
very varying order into another ValueObject which has the same
properties, but all of them are `readonly`. I tried to create somehow
the same: https://3v4l.org/Vb7fE#v8.2.4 There are places where I
have an array with a mix of them and I want to access the `$id` or
`$axisOrientation`. I also have methods on them. I know an
abstract class could do this for me but I feel that this would be the
most accurate way to define what I want.


[Example]
```php
interface Vehicle {
    public int $serialNumber;

    public function drive(): void;
}

/*
 * This would lead to an error, something like:
 * Class Car contains 1 abstract property and must therefore be
 * declared abstract or implement the remaining property
 * (Vehicle::$serialNumber)
 */
class Car implements Vehicle {
    public function drive() : void { }
}

// This would lead to an error too, because the type is different.
class Bike implements Vehicle {
    public string $serialNumber;

    public function drive() : void { }
}

// This would lead to an error too. The rule for readonly properties
// would be the same as right now between normal classes.
class Bike implements Vehicle {
    public readonly string $serialNumber;

    public function drive() : void { }
}
```


[Implementation]
Personally I don=E2=80=99t have experience in php internal development.
I do have C++ / some C experience. I'm going through the PHP
Internals book and every possible tutorial I find. I definitely would
like to take part in it, and I'm planning to do more later.

--=20
Bests

Zolt=C3=A1n Fekete

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