Read-only properties in PHP

PHP does not yet have support for read only properties, unlike TypeScript, which has the readonly modifier. When working with Value Objects I often find immutability a beneficial characteristic. To implement readonly properties in PHP I have a simple trait that I lean on.

The trait leverages the __get() magic method and PHPdoc property hints for IDE auto-completion. Below is an example class using the trait:

use App\Traits\ReadOnlyProperties;

/**
 * @property-read SheetMaterialSpec $materialSpec
 * @property-read float $length
 * @property-read float $width
 * @property-read int $quantity
 */
class Piece
{
    use ReadOnlyProperties;

    private float $length;
    private float $width;
    private int $quantity;

    public function __construct(float $length, float $width, int $quantity)
    {
        $this->length = $length;
        $this->width = $width;
        $this->quantity = $quantity;
    }
}

And this is trait:

namespace App\Traits;

use InvalidArgumentException;

trait ReadOnlyProperties
{
    public function __get($name)
    {
        if (property_exists($this, $name)) {
            return $this->$name;
        }

        if (isset($this->nullProperties) && in_array($name, $this->nullProperties)) {
            return null;
        }

        throw new InvalidArgumentException("Property $name does not exist on ".static::class);
    }
}

For bonus points, I even have a little unit test to verify the behaviour.