Voter example

votes.php

<?php

//Just a simple relationship
class Voter {
    private $id;
    private $votedTo;
    private $votedAt;

    public function votesTo(Party $party) {
        if ($this->votedTo) {
            throw new DomainException('Already voted');
        }
        $this->votedTo = $party;
        $this->votedAt = new Datetime();
    }
}

//With a value object
class Voter {
    private $id;
    private $vote;

    public function votesTo(Party $party) {
        if ($this->vote) {
            throw new DomainException('Already voted');
        }
        $this->vote = new Vote($party);
    }
}

class Vote {
    private $createdAt;
    private $toParty;

    public function __construct(Party $party) {
        $this->toParty = $party;
        $this->createdAt = new DateTime();
    }
}