Retain select menu value when error occurs

[Eiolon]

I am able to retain the value a user has entered into a text box in the event of an error.  How would I do this with a select menu?

 

Here is my select menu:

 

<select name="department_id" id="department_id" style="width:406px;"><option>SELECT</option>
<?php while($row_departments = mysql_fetch_array($departments)) { print '<option value="' . $row_departments['id'] . '">' . $row_departments['name'] . '</option>';}?>
</select>

 

I am not sure where to echo the selection at.

 

<?php if (isset($_POST[department_id])) echo $_POST[department_id]; ?>

 

[o3d]

<?PHP
//...
print '<option value="' . $row_departments['id'] . '" '.(isset($_GET['department_id'])&&$_GET['department_id']==$row_departments['id']?'selected':'').'>' . $row_departments['name'] . '</option>';
//...
?>