example.php

<?php

class Foo extends Freezable {
  
}

$dto = new Foo();
$dto->somProp = 'someVal';

Freezable::freeze($dto);

$dto->somProp = 'override'; // RuntimeException

Freezable.php

<?php

abstract class Freezable 
{

    private $attributes;
    private $isFreezed;

    public function __get($name) {
         return $this->attributes[$name];
    }

    public function __set($name, $value) {
        if ($this->isFreezed) throw new RuntimeException(‘DTO is freezed!’);
        $this->attributes[$name] = $value;
    }

    public static function freeze(Freezable $dto) {
         $dto->isFreezed = true;
    }
}