Immutable bidirectional reference

immutable_bidirectional_ref.php

<?php
declare(strict_types=1);

// https://softwareengineering.stackexchange.com/questions/334305/how-to-model-a-circular-reference-between-immutable-objects-in-c

final class Department {
    /** @var string */
    private $name;
    /** @var Closure|User */
    private $user;

    public function __construct(string $name, Closure $userResolver) {
        $this->name = $name;
        $this->user = $userResolver;
    }

    public function getName(): string {
        return $this->name;
    }

    public function getUser(): User {
        return $this->user;
    }

    public function resolveUser(User $user): self {
        if ($this->user instanceof Closure) {
            $this->user = $this->user->call($this, $user);
        }
        return $this;
    }
}

final class User {
    /** @var string */
    private $name;
    /** @var Department */
    private $department;

    public function __construct(string $name, Department $department) {
        $this->name = $name;
        $this->department = $department->resolveUser($this);
    }

    public function getName(): string {
        return $this->name;
    }

    public function getDepartment(): Department {
        return $this->department;
    }
}

$department = new Department('IT', function (User $user) {
    return $user;
});

$user = new User('John', $department);

echo sprintf(
    'User %s works in the %s department',
    $department->getUser()->getName(),
    $user->getDepartment()->getName()
);