Cannot instantiate a read-only class which extends another read-only class in PHP?

This is my class hierarchy:

readonly class ParentClass {
    public function __construct(
        public string $lastName
    ){
    }
}

readonly class ChildClass extends ParentClass {
    public function __construct(
        public string $firstName,
        public string $lastName
    ){
        parent::__construct($lastName);
    }
}

$child = new ChildClass('John', 'Doe');

I got an exception with message that "Cannot modify readonly property ParentClass::$lastName".

I’ve read many articles or answers of questions in Stackoverflow but nobody talked about this case or I didn’t find out.

Why did I got that error?

>Solution :

You are trying to redefine the property in the child class. You can’t do that.

Define the property only in the parent class.

readonly class ParentClass {
    public function __construct(
        public string $lastName
    ){
    }
}

readonly class ChildClass extends ParentClass {
    public function __construct(
        public string $firstName,
        string $lastName // No promoted property here. 
    ){
        parent::__construct($lastName);
    }
}