Cannot Modify Readonly Property

Why This Happens

PHP 8.1 introduced readonly properties that can only be assigned once during initialization. After the constructor sets a readonly property, any attempt to modify it throws this error. This enforces immutability for specific properties.

The Problem

class User {
    public function __construct(
        public readonly string $email
    ) {}
}
$user = new User('old@example.com');
$user->email = 'new@example.com'; // Cannot modify readonly

The Fix

class User {
    public function __construct(
        public readonly string $email
    ) {}

    public function withEmail(string $email): self {
        return new self($email);
    }
}
$user = new User('old@example.com');
$user = $user->withEmail('new@example.com');

Step-by-Step Fix

1. 1

   Understand readonly semantics

   Readonly properties are intentionally immutable. They can only be set in the class constructor or property declaration.

2. 2

   Use a wither method

   Create a method that returns a new instance with the modified value, following the immutable object pattern.

3. 3

   Remove readonly if mutability is needed

   If the property genuinely needs to change after construction, remove the readonly modifier and add validation in a setter instead.