PHP Constructor Property Promotion (PHP 8)

Before PHP 8, a class with three properties repeated each name three times: once in the property list, once as a parameter and once in the assignment.

index.php

PHP

<?php
class UserOld
{
    private string $name;
    private string $email;
    private bool $active;

    public function __construct(string $name, string $email, bool $active = true)
    {
        $this->name = $name;
        $this->email = $email;
        $this->active = $active;
    }

    public function name(): string
    {
        return $this->name;
    }
}

echo (new UserOld("Isha", "i@example.com"))->name();

Output

OUTPUT

Isha

Constructor property promotion (PHP 8.0) lets you write the visibility in front of the parameter. PHP then declares the property and assigns it for you.

index.php

PHP

<?php
class User
{
    public function __construct(
        private string $name,
        private string $email,
        private bool $active = true,
    ) {}

    public function name(): string
    {
        return $this->name;
    }
}

echo (new User("Isha", "i@example.com"))->name();

Output

OUTPUT

Isha

Same class, much shorter. The empty braces {} mean the constructor has nothing else to do.

Rules

A promoted parameter needs a visibility keyword (public, protected, private), readonly, or both.
You can mix promoted and normal parameters.
Default values work as usual.
A trailing comma after the last parameter is allowed (and recommended).

With readonly

index.php

PHP

<?php
class Point
{
    public function __construct(
        public readonly float $x = 0,
        public readonly float $y = 0,
    ) {}
}

$p = new Point(1.5, 2.5);

echo $p->x . ", " . $p->y;

Output

OUTPUT

1.5, 2.5

Extra logic in the constructor

You can still write code in the body. The promoted properties are already set when it runs.

index.php

PHP

<?php
class Email
{
    public function __construct(public readonly string $address)
    {
        if (!filter_var($this->address, FILTER_VALIDATE_EMAIL)) {
            throw new InvalidArgumentException("Invalid email");
        }
    }
}

try {
    new Email("nope");
} catch (InvalidArgumentException $e) {
    echo $e->getMessage();
}

Output

OUTPUT

Invalid email

Promoted and normal properties together

index.php

PHP

<?php
class Cart
{
    private array $items = [];   // a normal property

    public function __construct(private string $owner) {}   // promoted

    public function add(string $item): void
    {
        $this->items[] = $item;
    }

    public function summary(): string
    {
        return $this->owner . " has " . count($this->items) . " item(s)";
    }
}

$cart = new Cart("Ana");
$cart->add("Pen");

echo $cart->summary();

Output

OUTPUT

Ana has 1 item(s)

Limits

Not allowed in abstract constructors or in interfaces.
A promoted property cannot be of type callable.
You cannot declare the same property again in the class body.

With named arguments

Promotion and named arguments work very well together.

index.php

PHP

<?php
class Connection
{
    public function __construct(
        public string $host = "127.0.0.1",
        public int $port = 3306,
        public bool $ssl = false,
    ) {}
}

$c = new Connection(ssl: true);

echo "{$c->host}:{$c->port} ssl=" . ($c->ssl ? "yes" : "no");

Output

OUTPUT

127.0.0.1:3306 ssl=yes

lightbulb

The default shape of a modern class

Most new classes look like this: public function __construct(private readonly Dependency $dependency) {}. Short, typed and safe.

info

Why this matters in Laravel

Controllers and services in Laravel use promotion for dependency injection: public function __construct(private UserRepository $users) {}. That one line declares the property and tells Laravel what to supply.