Dafaq

errors.php

<?php
trait WithTrait
{
  public function with($key, $value) {
        if (false === array_key_exists($key, get_class_vars(self::class))) {
            sprintf(
                'Invalid key called. Key %s does not exist in class %s',
                $key,
                self::class
            )
        }
        $new = clone $this;
        $new->$key = $value;
        
        return $new;
    }
}
class Foo 
{
    protected $title;
    
    use WithTrait; 
    /**
     * Essentially ends up at
     * Invalid key called. Key title does not exist in class WithTrait
     */
}

FooHydratort.php

<?php
/**
 * @author Manuel Stosic <manuel.stosic@krankikom.de>
 */

namespace DrWolff\EventManagement\Hydrator\Seminar;

use DrWolff\EventManagement\Entity\Seminar\CoreData;
use Zend\Stdlib\Hydrator\HydratorInterface;

final class FooHydrator implements HydratorInterface
{
    /**
     * Hydrate $object with the provided $data.
     *
     * @param  array $data
     * @param  object $object
     * @return object
     */
    public function hydrate(array $data, $object)
    {
        foreach ($data as $key => $value) {
            $object = $object->with($key, $value);
        }

        return $object;
    }

    /**
     * Extract values from an object
     *
     * @param  CoreData $object
     * @return array
     */
    public function extract($object)
    {
        return [
            'id'                           => $object->getId(),
            'title'                        => $object->getTitle(),
            'description'                  => $object->getDescription(),
            // etc.....
        ];
    }
}

RelevantFormPart.php

<?php
final class FooFieldset extends Fieldset
{
    public function __construct($name = 'foo-fieldset', $options = [])
    {
        parent::__construct($name, $options);
    }

    public function init()
    {
        $this->setHydrator(new FooHydrator());
        $this->setObject(new Foo());
    }
}

works.php

<?php
class Foo 
{
    protected $title;
    
    public function with($key, $value) {
        if (false === array_key_exists($key, get_class_vars(self::class))) {
            sprintf(
                'Invalid key called. Key %s does not exist in class %s',
                $key,
                self::class
            )
        }
        $new = clone $this;
        $new->$key = $value;
        
        return $new;
    }
}