PHP Interfaces and Abstract Classes

<?php
interface Cache
{
    public function get(string $key): ?string;
    public function set(string $key, string $value): void;
}

final class ArrayCache implements Cache
{
    private array $items = [];

    public function get(string $key): ?string
    {
        return $this->items[$key] ?? null;
    }

    public function set(string $key, string $value): void
    {
        $this->items[$key] = $value;
    }
}

function cachedGreeting(Cache $cache, string $name): string
{
    $key = "greet:$name";
    if ($hit = $cache->get($key)) {
        return "$hit (from cache)";
    }
    $value = "Hello, $name!";
    $cache->set($key, $value);
    return $value;
}

$cache = new ArrayCache();
echo cachedGreeting($cache, "Ada"), "\n";
echo cachedGreeting($cache, "Ada"), "\n";

<?php
abstract class Shape
{
    abstract public function area(): float;

    public function describe(): string
    {
        return sprintf("%s with area %.2f", static::class, $this->area());
    }
}

class Circle extends Shape
{
    public function __construct(private float $r) {}
    public function area(): float { return M_PI * $this->r ** 2; }
}

class Rectangle extends Shape
{
    public function __construct(private float $w, private float $h) {}
    public function area(): float { return $this->w * $this->h; }
}

foreach ([new Circle(1), new Rectangle(2, 3)] as $shape) {
    echo $shape->describe(), "\n";
}