PHP interfaces, traits, and abstract classes

PHP supports single inheritance, so a class has only one parent. An interface lists method signatures that implementing classes must provide, and a class can implement many interfaces. A trait is a reusable set of methods that is copied into a class with use. It is the way to share code between classes that are not related by inheritance.

PHP

<?php
interface Shape
{
    public function area(): float;
}

trait Describes
{
    public function describe(): string
    {
        return static::class . " with area " . number_format($this->area(), 2);
    }
}

abstract class Base implements Shape
{
    use Describes;
}

final class Circle extends Base
{
    public function __construct(private float $radius) {}

    public function area(): float
    {
        return M_PI * $this->radius ** 2;
    }
}

final class Rect extends Base
{
    public function __construct(private float $width, private float $height) {}

    public function area(): float
    {
        return $this->width * $this->height;
    }
}

foreach ([new Circle(1.5), new Rect(2, 3.5)] as $shape) {
    echo $shape->describe(), "\n";
}
echo (new Circle(1)) instanceof Shape ? "a Circle is a Shape" : "not a Shape", "\n";

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