Variable in an array

I have this api in which it works with array and I want to pass a value that is defined in a variable and it does not take it, but neither does it throw me any error:

<?php
include 'include/cont_db.php';
require_once ('sdk-mercadopago/lib/mercadopago.php');

$code = '5LGIm61xtGES';

$ress = $conn->query("SELECT * FROM reservas WHERE code = '".$code."'");
$row = $ress->fetch();

$valor = (float)$row['valor']; //covierte dato de valor string a num

$cliente_id = "numero";
$cliente_secret = "numeros";

$mp = new MP($cliente_id, $cliente_secret);

$preference_data = array(
"items" => array(
    array(
        "title" => "Pago de Reserva",
        "quantity" => 1,
        "currency_id" => "ARS",
        "unit_price" => $valor //velor en pesos
    )
)
);

$preference = $mp->create_preference($preference_data);
?>
<h3><a href="<?php echo $preference['response']['init_point']; ?>">Pay</a></h3>

If I create a variable with the name titulo and replace the text, it works fine, but the same does not happen to me with the value.

[Solution]

Thanks a friend, I understood that the number that the value that was happening was of value string and "unit_price" must be a numerical value, that's why the payment link was not generated correctly. The solution was to convert the value string to num of the variable $valor making a typecast to (float) :

$valor = (float) $row['valor']; //covierte dato de valor string a num