class.nonReadOnly

Code example

<?php declare(strict_types = 1);

readonly class ParentClass
{
}

class ChildClass extends ParentClass
{
}

Why is it reported?

A non-readonly class cannot extend a readonly class. PHP enforces this constraint to maintain 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 all subclasses as well. Allowing a non-readonly child class would break this invariant because the child could introduce mutable properties.

In the example above, ChildClass is not readonly but extends the readonly ParentClass, which violates this rule.

How to fix it

Make the child class readonly as well:

 <?php declare(strict_types = 1);

 readonly class ParentClass
 {
 }

-class ChildClass extends ParentClass
+readonly class ChildClass extends ParentClass
 {
 }

Or remove the readonly modifier from the parent class if mutability is needed:

 <?php declare(strict_types = 1);

-readonly class ParentClass
+class ParentClass
 {
 }

 class ChildClass extends ParentClass
 {
 }