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+---
+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:
+
+```php
+class AccountDetails {
+
+  public function __construct(
+    public readonly string $accountNumber,
+    public readonly string $sortCode,
+  ) {}
+
+}
+```
+
+This means the public properties can be read and used without the need for getter methods, but cannot be overridden - making the DTO immutable.
+
+Without `readonly`, a DTO can be created and the property values can be changed:
+
+```php
+$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