Prestashop function query to the template - sql

Question

I am in prestashop 1.6 and in classes/Cart.php i've tested and i can communicate with the template file.
In the query that i've made to return the expect result works on phpmyadmin, and works in cart.php to tpl file call, but only when i write the id number of the customer, and i want to work with the variable id customer (something with THIS or like prestashop way of work - (int)$id_customer - ).
Any help please!!!!!
//code for cart.php
public static function payLater($id_customer)
{
$sql = "SELECT ps_customer.pay_later
FROM ps_customer
LEFT JOIN ps_cart ON ps_customer.id_customer = ps_cart.id_customer
ORDER BY ps_cart.id_cart DESC LIMIT 1
WHERE ps_cart.id_customer =".(int)$id_customer;
//if i change (int)$id_customer for a real id number like 43 it shows correct info
$result = Db::getInstance()->getValue($sql);
return $result;
}
//code for template file
{Cart::payLater(Tools::getvalue('pay_later'))}

Answer 1

{Cart::payLater($customer.id)} in tpl file. I have edited in your function also. please check below for empty customer id.
public static function payLater($id_customer)
{
if((int)$id_customer == 0){ return; } // check if customer id is blank
$sql = "SELECT ps_customer.pay_later
FROM ps_customer
LEFT JOIN ps_cart ON ps_customer.id_customer = ps_cart.id_customer
ORDER BY ps_cart.id_cart DESC LIMIT 1
WHERE ps_cart.id_customer =".(int)$id_customer;
//if i change (int)$id_customer for a real id number like 43 it shows correct info
$result = Db::getInstance()->getValue($sql);
return $result;
}