PHP - Associative Array

i have problem to get the sum from the associative array. i have to get sum for the 3 result that being display from the array

rsort($sub);
$a=0;
$arrlength = count($sub);
for($x = 0; $x < 3; $x++) {
  echo $sub[$x];
  

}

This is the cart array:
$cart = array( "1" => "2", "3" => "4", "5" => "6");
var_dump($cart);
array(3) { [1]=> string(1) "2" [3]=> string(1) "4" [5]=> string(1) "6" }
This is the cart items array:
$cart_items = array();
$cart_items["id"] = $id;
$cart_items["quantity"] = $quantity;
var_dump($cart_items);
array(2) { ["id"]=> array(2) { [0]=> string(1) "1" [1]=> string(1) "1" } ["quantity"]=> array(2) { [0]=> string(1) "1" [1]=> string(1) "1" } }

I want the key to be "id" and the value to be "quantity" like it is for the cart array. How do I define my cart_items array using variables?

$x = ['a' => 2, 'b' => 1, 'c' => 4, 'd' => 1];

$x1 = array_keys($x);
$x2 = array_values($x);

$z = [];
for ($i=0; $i<count($x); $i++) {
  array_push($z, [$x1[$i],$x2[$i]] );
} 

print_r($z); //[['a', 2], ['b', 1], ['c', 4], ['d', 1]];

(
[0] => ('id', '2')
[1] => ('name', 'Jhon')
)

function redeem() {
    // Check for required parameters
        $json = file_get_contents('php://input');
        $obj = json_decode($json,true);
        // Assumes $obj == array(0 => $assocArray, 1 => $assocArray)
        foreach($obj as $index => $assocArray) {
            // Assumes $assocArray == array(0 => array('id' => '2'), 1 => array('name' => 'John'))
            $stmt = $this->db->prepare('INSERT INTO prueba (id,nombre) VALUES (%d,%s)',$assocArray[0]['id'],$assocArray[1]['name']) or die(mysqli_error($this->db));
            $stmt->execute();
            
        }    
        
    }