populate text boxes with data from a dynamic mysql pull-down menu

I am trying to populate the text fields once a user selects an entry from the first pull-down menu.  I am using php and have created two files.  The first file (so far) just containts the form, similar to this (left out much detail for brevity):

<?php
session_start();
...
$query = "SELECT firstname, lastname, role FROM userdb";
$all_users = mysql_query($query,$mysql);
?>
...
some html
...
Select a user from the pulldown:
<form method='POST' action='moduser.php'>
<select name='userid'>
<?php
     while ($data = mysql_fetch_array($all_users, MYSQL_ASSOC)) {
          echo "<option value='{$data['username']}'>{$data['firstname']}</option>\n";
     }
?>
</select>
Firstname:
<input type='text' name='firstname'>
Username:
<input type='text' name='username'>
Role:
<select name="role">
  <option value="user">User</option>
  <option value="admin">Admin</option>
</select>
<input type='submit' value='Modify'>
</form>
...
</html>

The second file, moduser.php, is basically just an error checking php script that will update the fields in the database after the user submits the form.

However, I seem to be stuck at this point.  I don't know what steps to take to allow php to update all the text entries (and select entry) in the form, once someone selects a username from the pulldown menu.  I'm also using mysql to populate the first pull-down menu of usernames.

Any help is greatly appreciated...

Thanks!
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