<form action="Display.php" method="post">
  <select name="searchValue">
    <option value="<?php $allColomns ?>">All</option>
    <option value="itemCode">Item Code</option>
    <option value="shopNumber">Shop Code</option>
  </select>
  <input type="text" name="search">
  <input type="submit">
</form>

$search = "%" . $_POST["search"] . "%";
$searchValue = $_POST["searchValue"];
//create the query
$result = mysql_query("SELECT shopCode, itemNumber, itemDescription FROM sbh_itemsheet WHERE '$searchValue' LIKE '$search'");

 //return the array and loop through each row
while ($row = mysql_fetch_array($result))
{
?>
   <tr>
    <td><?php echo $row['id'];?></td>
    <td><?php echo $row['shopCode'];?></td>
    <td><?php echo $row['itemNumber'];?></td>
    <td><?php echo $row['itemDescription'];?></td>
  </tr>

$allColomns

$allcolomns = "shopCode, itemNumber, itemDescription";

$result = mysql_query("select * from sbh_itemsheet WHERE <column_name> LIKE '%".$_POST[search]."%'");