Fill dropdown with php array from arrays

Hi there,
​
I've been trying to fill a dropdown, unfortunately without success. First I've tried to fill it with data from a Read Data action, but I need to join tables which isn't possible. So I moved on to writing a query in php.
​

​

$db = JFactory::getDbo();
$query = $db->getQuery(true);

​
QUERY
​
​

$db->setQuery((string)$query);
return $db->loadAssocList();
​
​
​


​
​
In the debugger I get this:

[php12] => Array
        (
            [returned] => Array
                (
                    [0] => Array
                        (
                            [subscription_id] => 400000008
                            [subscription_name] => Jaarabonnement - Om de week 60 minuten (Volwassenen)
                            [course_name] => Zangles
                        )

                    [1] => Array
                        (
                            [subscription_id] => 400000001
                            [subscription_name] => Jaarabonnement - Wekelijks 30 minuten
                            [course_name] => Elektrisch Gitaarles
                        )

                )

            [var] => Array
                (
                    [0] => Array
                        (
                            [subscription_id] => 400000008
                            [subscription_name] => Jaarabonnement - Om de week 60 minuten (Volwassenen)
                            [course_name] => Zangles
                        )

                    [1] => Array
                        (
                            [subscription_id] => 400000001
                            [subscription_name] => Jaarabonnement - Wekelijks 30 minuten
                            [course_name] => Elektrisch Gitaarles
                        )

                )

        )

Inside my dropdown/select field I get the data with: {var:php12}
​
​
Now what happens is that my dropdown is filled with: Array, Array.
I want it like this: (value subscription_id) course_name - subscription_name. So I want course_name - subscription_name to be viewed in the dropdown and I want the subscription_id as value to use somewhere else.
​
What am I doing wrong and how can I get the result I want?
​
I hope my question and examples are clear enough.
​
Kind regards,
Nando