array_reduce to use dynamic variables pass in second function

I have below $test array

        Array
(
    [0] => Array
        (
            [quantity] => 3
            [stock_id] => _PHONE
        )

    [1] => Array
        (
            [quantity] => 3
            [stock_id] => 102
        )

    [2] => Array
        (
            [quantity] => 4
            [stock_id] => _PHONE
        )

    [3] => Array
        (
            [quantity] => 3
            [stock_id] => 102
        )

    [4] => Array
        (
            [quantity] => 4
            [stock_id] => _PHONE
        )

    [5] => Array
        (
            [quantity] => 6
            [stock_id] => _PHONE
        )

    [6] => Array
        (
            [quantity] => 2
            [stock_id] => 102
        )

)

and to sum same stock_id keys to one, i use below functions:

function sum($array, $key){
    isset($array[$key['stock_id']]) ? $array[$key['stock_id']]['quantity'] += $key['quantity'] : $array[$key['stock_id']] = $key;  
    return $array;
};

//merge same stock_id and sum the quantity same stock id
$sum_same_stock_id = array_reduce($test, "sum"); 

and the result went well like below:

$sum_same_stock_id:

Array
(
    [_PHONE] => Array
        (
            [quantity] => 17
            [stock_id] => _PHONE
        )

    [102] => Array
        (
            [quantity] => 8
            [stock_id] => 102
        )

)

So the question here is, I wanted to pass an dynamic keys values not just fixed values stock_id & quantity in sum function above. Have tried variety ways but still can't figured out the way. And can we put those functions into class as well?

Any advise is appreciated!