组合框的onchange事件

Question

I have a combobox which has a list of bank names and i am fetching it from DB.

I have one more combo box or a text box in which i want to display bank code. This bank code has to fill automatically for a corresponding bank name selected in 1st combo box.

i tried doing this, but its not working. Any help wil be greatly appreciated.

<label for="txtbankname"> Bank Name <em>*</em> </label>
<?php
  include_once 'dbconnection.php';
  $query1  = "SELECT BankName,BankId FROM bank_details";
  $result1 = mysqli_query($connection, $query1);

  echo "<select name='txtbankname'>";
  while ($result2 = mysqli_fetch_assoc($result1)) {        
  echo "<option value='" . $result2['BankId'] . "'>" . $result2['BankName'] . "</option>";        
}

  $bkcode = $result2['BankId'];
  echo $bkcode;
  echo "</select>";
?>

<label for="txtbankcode"> Bank Code <em>*</em> </label> 
<?php
  include_once 'dbconnection.php';
  $q2  = "SELECT BANKCODE FROM BANK_DETAILS ";
  $rs1 = mysqli_query($connection, $q2);

  echo "<select name='txtbankcode'>";
  while ($rs2 = mysqli_fetch_assoc($rs1)) {         
    echo "<option value='" . $rs2['BANKCODE'] . "'>" . $rs2['BANKCODE'] . "</option>";        
}
echo "</select>";

?>