Multi field selection in php

I have a product table with product code, description,selling price, cost,category etc. I have a drop down to select the product which also shows the description and the product price.
I would like to capture the product price and show/update it in a field when ever the user selects a product. on change the field should also change with the new value. How can I achieve this. I have attached my code for your kind perusal.
include('function/db_connect.php');
//session_start();
$price = Null;
//$items = Null;
if($_POST)
{
$var1 = $_POST['country'];
$var2 = $_POST['type'];
 if ($var1 != '') {        
       echo "<label>Item<span style='color:red'>*</span></label><span style='color:red'></span></label><span class='address'>";
       echo "<select id='items' name='items' style = 'width: 546px;' onChange='get_price(this.value)'>";          
       $sql = "SELECT * FROM tbl_product WHERE country_id = '$var1' AND type_id = '$var2'";  
       $db  = new DB_CONNECT();    
       $result = mysql_query($sql);
       $myarray = array();
       echo "<option value=''>Select</option>";
       while ($row = mysql_fetch_array($result)) {
            $idp = $row["product_id"];
            $iddes = $row["product_desc"];
            $selp = $row["product_sell"];
            $costp = $row["product_cost"];


       echo "<option value='" . $idp . "'>" . $iddes . "==>".$selp ."</option>";

       }
       echo "</select><label>Item</label></span><span class='address'>";

    }  
       echo "</div>";        
       echo "<br>";    



       echo "<div><label></label><span class='name'><button name = 'data'  type ='button' onclick= 'valprice('items')'>Validate value</button></span></div>";      

    }
?>