Demonstrates the use of traits and interfaces to "mock" multiple inheritance in PHP

interface-trait-inheritance.php

interface AsNumber {
	public function asNumber(): int;
}

interface CanBeAdded {
	public function add(AsNumber $b): int;
}

interface CanBeMultiplied {
	public function multiply(AsNumber $b): int;
}

/** @psalm-require-implements AsNumber */
trait NumberAdd {
	public function add(AsNumber $b): int{
		return $this->asNumber() + $b->asNumber();
	}
}

/** @psalm-require-implements AsNumber */
trait NumberMultiply {
	public function multiply(AsNumber $b): int{
		return $this->asNumber() * $b->asNumber();
	}
}

class RandomNumber implements AsNumber, CanBeAdded {
	use NumberAdd;

	public function asNumber(): int{
		return random_int(1, 10);
	}
}

class FixedNumber implements AsNumber, CanBeMultiplied, CanBeAdded {
	use NumberAdd;
	use NumberMultiply;

	/** @var int  */
	private $number;

	public function __construct(int $number){
		$this->number = $number;
	}

	public function asNumber(): int{
		return $this->number;
	}
}

$random = new RandomNumber;
$fixed = new FixedNumber(5);

echo $random->add($fixed)."\n";
echo $fixed->multiply($random);