PHP 8.2’s Readonly Properties: How They Can Improve Your Code

PHP 8.2 continues to introduce features that not only improve performance but also enhance the readability and maintainability of your codebase. One of the standout additions is Readonly Properties, a feature designed to enforce immutability at the property level.

Immutability is a core principle in modern software development. It ensures that once an object’s state is set, it cannot be modified, leading to more predictable and reliable code. In this article, we’ll explore the practical benefits of Readonly Properties, how to use them, and scenarios where they truly shine.

What Are Readonly Properties?

Readonly Properties are class properties that can be initialized once and cannot be changed thereafter. They are declared using the readonly keyword introduced in PHP 8.2.

Here’s a simple example:

class User {
    public readonly string $name;

    public function __construct(string $name) {
        $this->name = $name;
    }
}

$user = new User('John Doe');
// The following will throw an error:
// $user->name = 'Jane Doe';        

In this example, the $name property can only be set during object construction. Any subsequent attempt to modify it will result in a runtime error.

Benefits of Readonly Properties

1. Improved Code Clarity: Declaring properties as readonly makes your intent clear to other developers: these properties are not meant to change.
2. Encapsulation Without Boilerplate: Previously, achieving immutability required private properties and getter methods, adding boilerplate code. Readonly Properties simplify this:

class Order {
    public function __construct(
        public readonly int $id,
        public readonly float $amount
    ) {}
}

$order = new Order(1, 99.99);
// No need for additional getters or private properties.        

1. Enhanced Debugging:Bugs caused by unintended modifications to an object’s state are eliminated, reducing debugging time and effort.
2. Thread Safety and Concurrency:In applications involving multithreading or parallelism, immutability ensures data consistency and thread safety.

Common Use Cases for Readonly Properties

1. Value Objects: Value objects, such as coordinates, currency, or dates, benefit greatly from immutability.

class Coordinate {
    public function __construct(
        public readonly float $latitude,
        public readonly float $longitude
    ) {}
}

$location = new Coordinate(40.7128, -74.0060);
// $location->latitude = 42.0; // Error!        

1. DTOs (Data Transfer Objects):Readonly properties are perfect for DTOs, which represent immutable data structures passed between layers.

class ProductDTO {
    public function __construct(
        public readonly int $id,
        public readonly string $name,
        public readonly float $price
    ) {}
}        

1. Configuration Objects:Immutable configuration objects ensure that application settings remain consistent throughout execution.

class Config {
    public function __construct(
        public readonly string $dbHost,
        public readonly string $dbUser,
        public readonly string $dbPassword
    ) {}
}

$config = new Config('localhost', 'root', 'secret');
// $config->dbHost = 'newhost'; // Error!        

Limitations and Best Practices

While Readonly Properties are a powerful feature, they come with some limitations:

• Initialization Constraints: Readonly Properties can only be set once, and typically during object construction.
• Cannot Use Default Values: You must initialize a readonly property explicitly. The following will throw an error:

 class Example {
    public readonly int $value = 42; // Error!
}        

• Avoid Overuse: Use readonly properties where immutability is essential. Overusing them in mutable objects may limit flexibility unnecessarily.

Transitioning to Readonly Properties

If you're updating an existing codebase to PHP 8.2, consider the following steps:

1. Identify properties that are set only once during the lifecycle of an object.
2. Refactor these properties to use the readonly keyword.
3. Test thoroughly to ensure existing code doesn’t unintentionally attempt to modify readonly properties.

Conclusion

Readonly Properties in PHP 8.2 are a significant addition for developers aiming to write clean, predictable, and robust code. By enforcing immutability at the property level, they simplify class design and reduce the likelihood of bugs caused by unintended state changes.

Whether you’re building value objects, DTOs, or configuration classes, Readonly Properties offer a modern, streamlined approach to immutability in PHP.