Populate PHP array with data from database

Question

I have this kind of array which I want it to be populated with values from MySQL db. For example if there are 5 rows in the database, it should loop the array 5 times together with the values in it.

This is how my current array (to be parsed in JSON) looks like:

array(
    1 => array(
        "id" => 1,
        "category" => "For men",
        "items" => array(
            array(
                "id" => 1,
                "name" => "Baju",
                "price" => "5:52"
            ),
            array(
                "id" => 2,
                "name" => "The Canyon",
                "price" => "3:01"
            )
        )
    ),
    2 => array(
        "id" => 2,
        "category" => "Adele 21",
        "items" => array(
            array(
                "id" => 1, 
                "name" => "Rolling In The Deep", 
                "price" => "03:48"
            ),
            array(
                "id" => 2,
                "name" => "Rumour Has It",
                "price" => "03:43"
            )
        )
    )
),
.
.
.
. //more arrays here;

So I did a while loop to print out (print_r()) the array according from my database. It looks like this:

Array (
    [0] => Array (
        [id] => 1
        [item_photo] => gambar1
        [item_name] => Kotak
        [item_description] => Desc 1 - Lorem ipsum blablabla
        [user_id] => 6
        [category_id] => 2
    )
    [1] => Array (
        [id] => 2
        [item_photo] => gambar2
        [item_name] => Botol
        [item_description] => Desc 2 - Lorem ipsum blablabla
        [user_id] => 6
        [category_id] => 3
    )
    [n] . . .
    .
    .
    .
)

What I wanna achieve is to replace the values in the first array (to be parsed in JSON) with the one in array two. Like so,

array(
    $category_id => array(
        "id" => $i++,
        "category" => $category_name,
        "items" => array(
            array(
                "id" => $j++,
                "user_name" => $user_name,
                "name" => $item_name,
                "price" => $price,
                "item_description" => $item_description
                )
            )
        ),
); //notice the variables

This is only just a sketching of the idea of what I want to achieve. But I couldn't wrap my head around this to make it work.

Any idea how to populate the first array according to the second array's value?