Create PHP multidimension associative array from MySQL linked tables SELECT query

I'm hoping someone could help me with this problem.

Say I have 3 DB tables:

Users:
user_id, user_name
100, John
101, Jessica

Cars:
car_id, car_name
30, Corvette
31, BMW


UsersCars:
user_id, car_id, car_colour
100, 30, Red
101, 30, Green
101, 31, Green

(so John got a red corvette and Jessica has a green Corvette and a BMW)

I would like to have code that returns a multidimensional PHP array something like:

Array
(
    [100] => Array
    (
        [user_id] => 100
        [user_name] => John
        [cars] => Array
        (
            [car_id]=>30,
            [car_name]=>'Corvette',
            [car_colour]=>'Red'

        )            
    )
    [101] => Array
    (
        [user_id] => 101
        [user_name] => Jessica
        [cars] => Array
        (
            [0] => Array
            (
                [car_id]=>30,
                [car_name]=>'Corvette',
                [car_colour]=>'Green'
            ),
            [1] => Array
            (
                [car_id]=>31,
                [car_name]=>'BMW',
                [car_colour]=>'Green'
            )
        )            
    )
)

I have the following SQL

SELECT u.*, c.* FROM Users u
LEFT JOIN UsersCars uc ON u.user_id = uc.user_id
LEFT JOIN Cars c ON uc.car_id = c.car_id

And PHP

$result = mysqli_query($db, $q);
while ($row = mysqli_fetch_assoc($result)) {
    $users_with_cars[$row['user_id']] = $row;
}

But this isn't correct. Anyone knows how to solve this resulting the above array (with performance in mind)? I rather do not want to hard-coded the exception of "cars" being something that can happen more than once. I rather have something that just looks at $row and $users_with_cars and when seeing some new value, it appends it, by converting the old value into an array. Maybe there's already a native PHP function for this? Or better, maybe my MySQL or whole approach is wrong?

Any help or tips appreciated.

Regards

UPDATE SOLVED

Here's an update, maybe I can help someone else how I solved it eventually.

I ended up with always using an array for one or more cars, and I adjusted the tables to always have an "id" as column name. This way you can easily expand it. See example;

Users:
id, name
100, John
101, Jessica

Cars:
id, name
30, Corvette
31, BMW


UsersCars:
user_id, car_id, car_colour
100, 30, Red
101, 30, Green
101, 31, Green



$q = 'SELECT u.*, c.id as car_id, c.name as car_name, uc.colour as car_colour FROM Users u
LEFT JOIN UsersCars uc ON u.id = uc.user_id
LEFT JOIN Cars c ON uc.car_id = c.id';

$result = mysqli_query($db, $q);
while ($row = mysqli_fetch_assoc($result)) {
    $users_with_cars[] = $row;
}
$joins = array('cars' => array('car_id'=>'id','car_name'=>'name','car_colour'=>'colour'));
$users_with_cars = create_join_array($users_with_cars, $joins);

print_r($users_with_cars);



function create_join_array($rows, $joins){
    /* build associative multidimensional array with joined tables from query rows */

    foreach((array)$rows as $row){
        if (!isset($out[$row['id']])) {
            $out[$row['id']] = $row;
        }

        foreach($joins as $name => $item){
            unset($newitem);
            foreach($item as $field => $newfield){
                unset($out[$row['id']][$field]);
                if (!empty($row[$field]))
                    $newitem[$newfield] = $row[$field];
            }
            if (!empty($newitem))
                $out[$row['id']][$name][$newitem[key($newitem)]] = $newitem;
        }
    }

    return $out;
}

This all results in the beautiful array:

Array
(
    [100] => Array
    (
        [id] => 100
        [name] => John
        [cars] => Array
        (
            [30] => Array
            (
                [id]=>30
                [name]=>'Corvette',
                [colour]=>'Red'
            )
        )            
    )
    [101] => Array
    (
        [id] => 101
        [name] => Jessica
        [cars] => Array
        (
            [30] => Array
            (
                [id]=>30,
                [name]=>'Corvette',
                [colour]=>'Green'
            ),
            [31] => Array
            (
                [id]=>31,
                [name]=>'BMW',
                [colour]=>'Green'
            )
        )            
    )
)

Let's say the users also can have multiple bikes. Then, you have multiple join arrays, you can easily bind on with left joins and add it to the join array.

$q = 'SELECT u.*, c.id as car_id, c.name as car_name, uc.colour as car_colour, b.id as bike_id, b.name as bike_name FROM Users u
LEFT JOIN UsersCars uc ON u.user_id = uc.user_id
LEFT JOIN Cars c ON uc.car_id = c.id
LEFT JOIN UsersBikes ub ON u.user_id = ub.user_id
LEFT JOIN Bikes b ON ub.bike_id = b.id';

$result = mysqli_query($db, $q);
while ($row = mysqli_fetch_assoc($result)) {
    $users_with_cars_bikes[] = $row;
}

$joins = array('cars' => array('car_id'=>'id', 'car_name'=>'name', 'car_colour'=>'colour'), 
               'bikes' => array('bike_id'=>'id', 'bike_name'=>'name'));
$users_with_cars_bikes = create_join_array($users_with_cars_bikes, $joins);

print_r($users_with_cars_bikes);

Would result in something like

Array(
    [100] => Array
    (
        [id] => 100
        [name] => John
        [cars] => Array
        (
            [30] => Array
            (
                [id]=>30
                [name]=>'Corvette',
                [colour]=>'Red'
            )
        )
        [bikes] => Array
        (
            [41] => Array
            (
                [id]=>41
                [name]=>'BMX'
            )
        )           
    )
)

and so on..

Thanks all for helping out :)