Immutable read-only properties in PHP 8.1

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

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:

$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