Save the user_id of a logged in user when submitting a form in ZF2

I want to save the users user_id along the data he submits via a form. This is in my controller:

The controller:

$request = $this->getRequest();
        if ($request->isPost()) {
            $testing = new Testing();
            $form->setInputFilter($testing->getInputFilter());
            $form->setData($request->getPost());

            if ($form->isValid()) {
                $this->zfcUserAuthentication()->hasIdentity();  // user logged in?
                $user = $this->zfcUserAuthentication()->getIdentity(); //get identity
                $user_id = $user->getID(); //gets the user-id, output: number, eg. 18



                $testing->exchangeArray($form->getData());
                $this->getTestingTable()->saveTesting($testing);

                // Redirect to iaps test
                return $this->redirect()->toUrl('/testing/iaps4');
            }
        }

        return array('form' => $form);


    }

If I print_r($user_id) the output will be plain and simple 18. I tried several things that did not work, here the most promising ones:

• a seperate exchangeArray for the user_id only. Unfortunately, it only generates an empty entry in my database along the data form.
• combine the $testing->exchangeArray($form->getData()); with the user_id data, obviously no success here either.

The thing is, I kinda know what I need: the plain output 18 from the $user->getID(); won't help me, because I need to asign this number to the user_id column in the database along the data submitted by the user with the id user_id.

I'm using Zend Framework 2.3.3, ZF-Commons/ZfcBase 0.0.1 and ZF-Commons/ZfcUser 1.2.2

You can merge user_id with the data from your form in a single array with array_merge :

$testing->exchangeArray(array_merge(
    $form->getData(),
    ['user_id' => $user->getID()]
));