How to relate 2d PHP array data with a result set from CodeIgniter's query builder by primary key

I have variable array($avail) like this :

$avail = [
    ['item_id' => 1, 'qty_avail' => 44],
    ['item_id' => 4, 'qty_avail' => 33],
    ['item_id' => 6, 'qty_avail' => 50],
];

And I want select with CodeIgniter:

$this->db->select(
    'item_id,'
    . $avail[0]['qty_avail'] . ' as "qty_avail", 
        qty_tocome, qty_togo'
);
$this->db->order_by('item_id', 'asc');
$this->db->where('item_category_id = ', $category_id);
$query = $this->db->get('t_inventory i', $limit, $offset)->result_array();
print_r($query);

The result is all [qty_avail] always $avail[0] :

Array (
[0] => Array
    (
        [item_id] => 1
        [qty_avail] => 44
        [qty_tocome] => 0.00
        [qty_togo] => 0.00
    )

[1] => Array
    (
        [item_id] => 4
        [qty_avail] => 44
        [qty_tocome] => 0.00
        [qty_togo] => 0.00
    )

[2] => Array
    (
        [item_id] => 6
        [qty_avail] => 44
        [qty_tocome] => 0.00
        [qty_togo] => 0.00
    )
);

But I want the qty_avail value which relates to each item_id, like this:

Array (
[0] => Array
    (
        [item_id] => 1
        [qty_avail] => 44
        [qty_tocome] => 0.00
        [qty_togo] => 0.00
    )

[1] => Array
    (
        [item_id] => 4
        [qty_avail] => 33
        [qty_tocome] => 0.00
        [qty_togo] => 0.00
    )

[2] => Array
    (
        [item_id] => 6
        [qty_avail] => 50
        [qty_tocome] => 0.00
        [qty_togo] => 0.00
    )
);

You can use array_replace_recursive like this:

<?php
$arr = [
    ["item_id" => 1,
    "qty_tocome" => 0.00,
    "qty_togo" => 0.00],
    ["item_id" => 4,
    "qty_tocome" => 0.00,
    "qty_togo" => 0.00],
    ["item_id" => 6,
    "qty_tocome" => 0.00,
    "qty_togo" => 0.00]
];

$avail = [
["qty_avail" => 44],
["qty_avail" => 33],
["qty_avail" => 50]
];

$finalArr = array_replace_recursive($arr,$avail);
print_r($finalArr);

So your output will be:

Array
(
    [0] => Array
        (
            [item_id] => 1
            [qty_tocome] => 0
            [qty_togo] => 0
            [qty_avail] => 44
        )

    [1] => Array
        (
            [item_id] => 4
            [qty_tocome] => 0
            [qty_togo] => 0
            [qty_avail] => 33
        )

    [2] => Array
        (
            [item_id] => 6
            [qty_tocome] => 0
            [qty_togo] => 0
            [qty_avail] => 50
        )

)

But make sure that the quantity gets assigned in the right order, since you are using $this->db->order_by('item_id', 'asc');.