initializing $customer_id, where?

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I'm having to make some modifications to the shopping_cart.php class, but I can't figure out where customer_id is intialized. Here's an sql insert that uses $customer id:

if (tep_session_is_registered('customer_id')) tep_db_query("insert into " . TABLE_CUSTOMERS_BASKET . " (customers_id, products_id, customers_basket_quantity, customers_basket_date_added) values ('" . $customer_id . "', '" . $products_id . "', '" . $qty . "', '" . date('Ymd') . "')");

thanks in advance.