Unable to send variables to MySQL table

Hi folks... a bit of a newbie question I'm afraid... I've written this script shown below. It gets its variables from a form, and then it (supposedly!) writes these values into a MySQL table ('invoices'). The script executes with no errors, but when I check the table, the table is still empty. I can manually insert the data directly into the table, and when I echo the variables in the script, the values are displayed whe I run it, but for reasons unknown, the values are not written to the table. Any ideas? The code is below. Many thanks. Stewart <?php // this opens the connection to the db include 'library/opendb.php'; // this adds detals to the invoice table $item1_desc = $_REQUEST['item1_desc']; $item2_desc = $_REQUEST['item2_desc']; $item3_desc = $_REQUEST['item3_desc']; $item4_desc = $_REQUEST['item4_desc']; $item1_cost = $_REQUEST['item1_cost']; $item2_cost = $_REQUEST['item2_cost']; $item3_cost = $_REQUEST['item3_cost']; $item4_cost = $_REQUEST['item4_cost']; $delivery_cost = $_REQUEST['delivery_cost']; $add_to_db = "insert into invoices (item1_desc, item1_cost, item2_desc, item2_cost, item3_desc, item3_cost, item4_desc, item4_cost, delivery_cost) values ('$item1_desc', '$item1_cost', '$item2_desc', '$item2_cost', '$item3_desc', '$item3_cost', '$item4_desc', '$item4_cost', '$delivery_cost')"; mysql_query($add_to_db); ?>