The DTO has its validation, the controller uses the negotiation lib to get the proper formatter (twig or json_encode), the binder validate and fill the dto and create a BindResult

BindResult.php

class BindResult {

    private $data;
    private $errors;

    function __construct($data, $errors)
    {
        $this->data = $data;
        $this->errors = $errors;
    }

    /**
     *
     * @return mixed
     */
    public function getData()
    {
        return $this->data;
    }

    /**
     * @return array
     */
    public function getErrors()
    {
        return $this->errors;
    }

    /**
     * @return bool
     */
    public function isValid()
    {
        return empty($this->errors);
    }
}

page_create.twig.html

{# tutti i campi uno solo di esempio #}
{# render_select(explicit true/false, multiple true/false, $options, $selected,  htmlAttributes) #}
{% render_select(explicit true/false, multiple true/false, authorsFromController, bindResult.data.authors, ['class'=>'authors'] , bindResult.error.authors) %}

PageController.php

<?php
class PageController extends Controller {

    public function createAction(Request $request) {

        $page = null
        //$this->formatter->getFormat() //html or json or xml
        //$this->formatter->getHtmlTemplatePath() // UI/Resources/views/create_get.twig.html
        return $this->formatter->render($page);
    }


    public function postAction(Request $request) {
        //bind->isValid()
        //bind->getData()
        //bind->getErrors()
        $bindResult = $this->bind($request, Pages\DTO\CreatePage::class);

        if (!$bindResult->isValid() && $this->formatter->isHtml()) {
            return $this->renderHtml('pages_create', $bindResult, $authorsFromController);
        }

        return $this->handle($bindResult->getData()); // will call the CreatePageHandler
    }
}

POSTPage.php

<?php

namespace Pages\DTO;

use Symfony\Component\Validator\Constraints as Assert;

class POSTPage {

    /**
     * @Assert\NotBlank()
     * @Assert\Length(
     *      min = 2,
     *      max = 50)
     */
    private $title;

    /**
     * @Assert\NotBlank()
     * @Assert\Length(
     *      min = 10,
     *      max = 500)
     */
    private $body;

    /**
     * @Assert\NotBlank()
     * @Assert\Length(
     *      min = 2,
     *      max = 50)
     */
    private $seoTitle;

    /**
     * @Assert\NotBlank()
     */
    private $seoDescription;

    /**
     * @Assert\NotNull
     * @Assert\Type(
     *     type="array"
     * )
     */
    private $authors;

    private $published;

    function __construct($title, $body, $seoTitle, $seoDescription, $authors, $published)
    {
        $this->title = $title;
        $this->body = $body;
        $this->seoTitle = $seoTitle;
        $this->seoDescription = $seoDescription;
        $this->author = $author;
        $this->published = $published;
    }

    /**
     * @return mixed
     */
    public function getTitle()
    {
        return $this->title;
    }

    /**
     * @return mixed
     */
    public function getBody()
    {
        return $this->body;
    }

    /**
     * @return mixed
     */
    public function getSeoTitle()
    {
        return $this->seoTitle;
    }

    /**
     * @return mixed
     */
    public function getSeoDescription()
    {
        return $this->seoDescription;
    }

    /**
     * @return mixed
     */
    public function getAuthor()
    {
        return $this->author;
    }

    /**
     * @return mixed
     */
    public function getPublished()
    {
        return $this->published;
    }
}