Getting auto_increment value

[eluser]penta997[/eluser]
How can I add max value to another foreign key. this method doesn't work.

Code:

function set_order_books($id=1,$ilosc=1)
        {
            $ORDER_ID = $this->db->query("select MAX(ORDER_ID) from orders_ID")->row()->ORDER_ID;
            $BOOK_Price = $this->db->query("select BOOK_Price from books where BOOK_ID=$id")->row()->BOOK_Price;
             $order_books = array(

               'ORDERS_ORDER_ID' => $ORDER_ID,
               'BOOKS_BOOK_ID' => $id,
               'ORDERB_Price' => $BOOK_Price*$ilosc,
               'ORDERB_Quantity' => $ilosc
           );

                 $sob = $this->db->insert('order_books',$order_books);
                 return $sob;
        }

I have a php error

A PHP Error was encountered

Severity: 4096

Message: Object of class stdClass could not be converted to string

Filename: mysql/mysql_driver.php

Line Number: 535