Understanding Class Type Declarations in PHP

In PHP, class type declarations act as a validation mechanism that ensures methods receive objects of the expected class, preventing type‑mismatch errors.

The article uses a shopping‑cart analogy to illustrate how unwanted items (incorrect types) can slip into a collection if not properly checked.

It then introduces a simple CarShop example, showing a class with a $cars array and an addCar method that initially lacks type hints.

<?php

class CarShop
{
    public $cars = [];

    public function addCar($car): void 
    {
      $this->cars[] = $car;
    }
}

A Car class is defined with public properties $name and $color using a constructor.

<?php

class Car
{
    public function __construct(public string $name, public string $color)
    {
    }
}

Instances of Car are created and added to the CarShop, but a string (a “fake car”) is also added, demonstrating the problem when type declarations are missing.

<?php 

require_once 'CarShop.php';
require_once 'Car.php';

$car1 = new Car("Mercedes GLK", "red");
$car2 = new Car("Mercedes GLE", "green");
$car3 = "red Mercedes"; // not a Car object

$carShop = new CarShop();
$carShop->addCar($car1);
$carShop->addCar($car2);
$carShop->addCar($car3);

When iterating over $carShop->cars, PHP throws a warning because the third element is a string, not a Car object.

foreach ($carShop->cars as $car) {
    print $car->name;
}

To fix the issue, the addCar method is updated with a class type declaration addCar(Car $car): void, forcing only Car objects to be added.

<?php

class CarShop
{
    public $cars = [];

    public function addCar(Car $car): void
    {
        $this->cars[] = $car;
    }
}

Running the corrected code results in a fatal TypeError when a string is passed, demonstrating how class type declarations protect against invalid data.

Fatal error: Uncaught TypeError: CarShop::addCar(): Argument #1 ($car) must be of type Car, string given...

The article concludes that using class type declarations ensures type safety, improves code reliability, and prevents runtime errors caused by incorrect object types.