Procesar los resultados de SQL JOIN con PHP

I have an issue with processing data with PHP I retrieved from MySQL. This is the table schema:

parents:
id | firstname | lastname

children:
id | firstname | lastname

parent_child_link
child_id | parent_id

I need to save the data this way, because I'd like to link multiple parents to one child. Now when I want to retrieve a child, I'd like to get all the information of the child, but also the parent id's. This obviously is a JOIN query, so I use:

SELECT *
FROM children c
JOIN parent_child_link l on l.child_id=c.id
WHERE c.id=1

Now, when this child with id=1 has 2 parents (lets say with id's 1 and 2), I get this as a result:

id | firstname | lastname | parent_id
1    test        test       1
1    test        test       2

If I process this in PHP, I would get 2 arrays, but I want this in one array, e.g.

array(
    'firstname' => 'test', 
    'lastname' => test',
    'parents' => array(1, 2)
)

¿Cómo puedo conseguir esto?

¡Muchas gracias!