1_UserId.php

<?php

namespace Acme\Project\Model\User\ValueObject;

class UserId
{
    /**
     * @var string
     */
    private $id;

    /**
     * @param string $id
     */
    public function __construct($id)
    {
        Assert::guid($id, "Invalid user id.");

        $this->id = $id;
    }

    /**
     * @return string
     */
    public function __toString()
    {
        return $this->id;
    }
}

2_Username.php

<?php

namespace Acme\Project\Model\User\ValueObject;

class Username
{
    /**
     * @var string
     */
    private $value;

    /**
     * @param string $id
     */
    public function __construct($value)
    {
        Assert::string($value, "Invalid username.");
        Assert::betweenLength($value, 3, 21, "Username must be between 4 and 20 characters in length.");

        $this->value = $value;
    }

    /**
     * @return string
     */
    public function __toString()
    {
        return $this->value;
    }
}

3_User.php

<?php

namespace Acme\Project\Model\User\Entity;

class User
{
    /**
     * @var string
     */
    private $id;

    /**
     * @var Username
     */
    private $username;

    /**
     * @param UserId $id
     * @param Username $username
     */
    public function __construct(UserId $id, Username $username)
    {
        $this->id = (string)$id;
        $this->username = $username;
    }

    /**
     * @return UserId
     */
    public function getId()
    {
        return new UserId($this->id);
    }
}

4_User.orm.yml

Acme\Project\Model\User\Entity\User:
    type: Entity
    id:
        id:
            type: guid
            generator:
                strategy: NONE
    embedded:
        username:
            class: Acme\Project\Model\User\ValueObject\Username
            columnPrefix: false

5_Username.orm.yml

Acme\Project\Model\User\ValueObject\Username:
    type: embeddable
    fields:
        value:
            type: string
            column: username