class.readOnly

Code example

<?php declare(strict_types = 1);

class Nonreadonly
{
}

readonly class Foo extends Nonreadonly
{
}

Why is it reported?

A readonly class can only extend another readonly class, and a non-readonly class cannot extend a readonly class. PHP enforces this constraint to maintain consistency of the readonly invariant across the inheritance chain. When a class is declared as readonly, all its properties are implicitly readonly, and this guarantee must hold for parent classes as well.

In the example above, the readonly class Foo extends the non-readonly class Nonreadonly, which violates this rule.

How to fix it

Make the parent class readonly as well:

 <?php declare(strict_types = 1);

-class Nonreadonly
+readonly class Nonreadonly
 {
 }

 readonly class Foo extends Nonreadonly
 {
 }

Or remove the readonly modifier from the child class:

 <?php declare(strict_types = 1);

 class Nonreadonly
 {
 }

-readonly class Foo extends Nonreadonly
+class Foo extends Nonreadonly
 {
 }