oliverdavies.uk/source/_daily_emails/2023-04-13.md

---
title: >
    Immutable read-only properties in PHP 8.1
pubDate: 2023-04-13
permalink: >-
    archive/2023/04/13/immutable-read-only-properties-in-php-8-1
tags:
    - php
---

Continuing with yesterday's data transfer object (DTO) example, something that can be done since PHP 8.1 is to make properties read-only:

```language-php
class AccountDetails {

  public function __construct(
    public readonly string $accountNumber,
    public readonly string $sortCode,
  ) {}

}
```

This means the public properties can be read and used without the need for getter methods, but cannot be overridden - making the DTO immutable.

Without `readonly`, a DTO can be created and the property values can be changed:

```language-php
$accountDetails = new AccountDetails('12345678', '00-00-00');
$accountDetails->accountNumber = 'banana';
```

With `readonly` set, you'd get a fatal error instead:

> Fatal error: Uncaught Error: Cannot modify readonly property AccountDetails::$accountNumber in /home/opdavies/tmp/example.php:13