Test Data Builders are just normal Builders with default values used exclusively in your test suites so that you don't have to specify irrelevant parameters on specific test cases:

class AuthorBuilder
{
    private $username;
    private $email ;
    private $fullName;

    private function __construct()
    {
        $this->username = new Username('johndoe');
        $this->email = new Email('[email protected]');
        $this->fullName = new FullName('John', 'Doe');
    }

    public static function anAuthor()
    {
        return new self();
    }

    public function withFullName(FullName $aFullName)
    {
        $this->fullName = $aFullName;

        return $this;
    }

    public function withUsername(Username $aUsername)
    {
        $this->username = $aUsername;

        return $this;
    }

    public function withEmail(Email $anEmail)
    {
        $this->email = $anEmail ;

        return $this;
    }

    public function build()
    {
        return new Author($this->username, $this->fullName, $this->email);
    }
}

class MyTest extends PHPUnit_Framework_TestCase
{
    /**
     * @test
     */
    public function itDoesSomething()
    {
        $author = AuthorBuilder::anAuthor()
            ->withEmail(new Email('[email protected]'))
            ->build();
    }
}

We could even combine Test Data Builders to build more complicated Aggregates, like a Post:

class Post
{
    private $id;
    private $author;
    private $body;
    private $createdAt;

    public function __construct(
        PostId $anId, Author $anAuthor, Body $aBody
    ) {
        $this->id = $anId;
        $this->author = $anAuthor;
        $this->body = $aBody;
        $this->createdAt = new DateTimeImmutable();
    }
}

Let's see the corresponding Test Data Builder for our Post. We could reuse the AuthorBuilder for building a default Author:

class PostBuilder
{
    private $postId;
    private $author;
    private $body;

    private function __construct()
    {
        $this->postId = new PostId();
        $this->author = AuthorBuilder::anAuthor()->build();
        $this->body = new Body('Post body');
    }

    public static function aPost()
    {
        return new self();
    }

    public function withAuthor(Author $anAuthor)
    {
        $this->author = $anAuthor;

        return $this;
    }

    public function withPostId(PostId $aPostId)
    {
        $this->postId = $aPostId;

        return $this;
    }

    public function withBody(Body $body)
    {
        $this->body = $body;

        return $this;
    }

    public function build()
    {
        return new Post($this->postId, $this->author, $this->body);
    }
}

This solution is now flexible enough to cover any test case, including the possibility of building inner Entities:

class MyTest extends PHPUnit_Framework_TestCase
{
    /**
     * @test
     */
    public function itDoesSomething()
    {
        $post = PostBuilder::aPost()
            ->withAuthor(AuthorBuilder::anAuthor()
            ->withUsername(new Username('other'))
                ->build())
            ->withBody(new Body('Another body'))
                ->build();

        //do something with the post
    }
}