PHP 8.1 - Readonly Properties

If you regularly read the blog, you probably know that I like to write strongly typed code wherever possible, and use static analysis wherever I can.

You can imagine how happy I was when I first saw the readonly properties RFC pop-up for PHP 8.1: once again a feature that helps me write cleaner, properly typed code.

PHP 7.4 already gave us typed properties,

PHP 8.0 introduced promoted properties

Maybe a quick explainer is warranted about what readonly properties are: they are an easy way to only allow a property value to be set once, and prohibit any changes to them from that point on. They are a great way to model value objects and data-transfer objects, which I use quite a lot!

Ok, that's enough rambling, let's get on with readonly properties!

Let me show you the difference it makes when all these features are combined. This is what a (simplified) DTO would look like before PHP 7.4:

class BlogData
{
    /** @var string */
    private $title;

    /** @var State */
    private $state;
    
    public function __construct(
        string $title,
        State $state
    ) {
        $this->title = $title;
        $this->state = $state;
    }
    
    public function getTitle(): string
    {
        return $this->title;    
    }
    
    public function getState(): State 
    {
        return $this->state;    
    }
}

PHP 7.4 added typed properties

class BlogData
{
    private string $title;

    private State $state;
    
    public function __construct(
        string $title,
        State $state
    ) {
        $this->title = $title;
        $this->state = $state;
    }
    
    public function getTitle(): string
    {
        return $this->title;    
    }
    
    public function getState(): State 
    {
        return $this->state;    
    }
}

Next

PHP 8.0 added constructor property promotion

class BlogData
{
    public function __construct(
    private string $title,
    private State $state,
    ) {}

    public function getTitle(): string
    {
        return $this->title;    
    }
    
    public function getState(): State 
    {
        return $this->state;    
    }
}

And now, finally, there's readonly properties as well, having the most significant impact of all:

class BlogData
{
    public function __construct(
    public readonly string $title,
    public readonly State $state,
    ) {}
}

I guess you'd understand when I tell you that readonly properties are my personal favourite feature of PHP 8.1.