Creating DTOs with Typed Properties

DTOs use typed, explicit fields to transfer validated values between layers.

What It Does

DTOs use typed, explicit fields to transfer validated values between layers. The following examples distinguish a normal use case, a boundary case and a practical application.

Requirements and Compatibility

PHP 8.x and a local test environment. Apply input validation, correct error handling and the requirements of any optional libraries.

The code focuses on the core concept. Production implementation may also need routes, validation, persistence, authorization or deployment tooling.

Example 1: Basic Behavior

Start with the smallest meaningful working case.

<?php
final class UserDto { public function __construct(public readonly int $id, public readonly string $email) {} }
print_r(new UserDto(5, '[email protected]'));

Example 2: Another Case

Examine the edge case and the observable result.

<?php
final class ProductDto { public function __construct(public readonly string $sku, public readonly int $priceCents) { if ($priceCents < 0) throw new InvalidArgumentException('Negative price'); } }
echo (new ProductDto('P01', 250))->sku;

Practical Example: Putting It to Use

Apply the technique to a small practical task.

<?php
final class ResponseDto { public function __construct(public readonly string $status, public readonly array $items) {} public function json(): string { return json_encode(['status' => $this->status, 'items' => $this->items], JSON_THROW_ON_ERROR); } }
echo (new ResponseDto('ok', [1,2]))->json();

Important Behavior and Edge Cases

A public readonly field prevents reassignment, not mutation of a referenced object inside it.

Comparison with Related Functions or Approaches

Common Beginner Mistakes

Key Takeaways

• DTOs use typed, explicit fields to transfer validated values between layers.
• A public readonly field prevents reassignment, not mutation of a referenced object inside it.
• Keep business validation in dedicated constructors or domain objects.

Before adapting these examples, verify the return values, test the failure paths and confirm that your application meets its security and environment requirements.