CakePHP 3.5.1 Validation (field update base on another field)

Question

Previously, I asked the following question. When I'am in the edit-screen and I set the value "Done" in the field "Project_status", is it then possible that I can force me to change the field "Afgehandeld" to the value "Arjan" automaticly.

Because I was not able to add my code to the provided answeres, I asked it again, but now with the code that I already have.

This code is unfortunily not working

Who can further help this newbie?

Thanks in advance.

<?php 
/* src/template/projecten/index.ctp */ 
        foreach ($projecten as $DB_inhoud):
        ...
        ...
        echo  $DB_inhoud->Project_status;
        echo  $DB_inhoud->Opgeleverd;
        ...
        ...
        endforeach;
?>



<?php 
/* src/template/projecten/edit.ctp */
    ...
    echo $this->Form->select('Project_status', $status, ['escape' => true]);
    echo $this->Form->input('Opgeleverd',['label' => false]);
    ...
    ...
?>


<?php
/* src/Model/Table/ProjectenTable.php */

namespace App\Model\Table;

use Cake\ORM\Table;
use Cake\Validation\Validator;
use Cake\ORM\RulesChecker;

Class ProjectenTable extends Table
{
    Public function initialize (array $config)
    {
        $this->addBehavior('Timestamp');
    }

    public function customValidationMethod($check, array $context)
    {

        $validator->add('Projecten->Opgeleverd', 'custom', 
            ['rule' => function ($value, $context) 
            {
                echo $context;
                if ($context['Projecten->Project_status'] == "100. Project opgeleverd") 
                    {
                        return $value == "Arjan";
                    }
                return true;
            },
            'message' => 'Error message'
            ]);
    }
}