Auto-Populate Text Field Based On DropDown List Selection

I have set up my website quite some time ago that has served its purpose very well, however I will now be adding an order form functionality. So far, by reading books and traversing forums, I have been able to develop a page where the user
1.Enters data into the required fields, that is then submitted to a MYSQL database via PHP,
2.Can retrieve orders that are stored in the database,
3.Delete orders that are stored in the database.

I have wamp installed on my computer as the webserver. I have also incorporated two drop down lists that both retrieve their values from tables within the database. The first drop down list retrieves the Australian States that I have stored in a table, and once the submit button is pressed, it stores the State that is selected to a separate table. This drop down list functions as it should. An extract from "From Place an order.php"

<?php
$dbcnx = @mysql_connect('localhost', 'root', 'tingling');
if (!$dbcnx) {
exit('<p>Unable to connect to the ' .
'database server at this time.</p>');
} .....

The second drop down list retrieves product names from a table that contains products and their prices. What I would like to happen, is that when the product is selected from this drop down list, a text box is automatically filled with its corresponding price. I had this drop down list working as per the "States" drop down list, but could not get it to auto populate the text field. I got some assistance from a friend and was able to get the text box to auto populate with its corresponding price, however when the page was submitted to the database, the "id" number of the product name from the drop down list was stored, and not the product name.

<select name="productSelection" onchange = "getProductDetails(this)">
<option selected value="1product">Select Product</option>
<?php
$products1 = @mysql_query('SELECT * FROM products');
if (!$products1) {
exit('<p>Unable to obtain author list from the database.</p>'); .....