posting a selection from a dropdown box to a variable

Hi-
This code displays a dropdown box containing a list of all my current clients.  I need the selected clients id number to post to $referedby.  

Any ideas?

$sql = "SELECT id, name FROM results";
$res_cat = mysql_query($sql); // handle to category set

<select name="clientid">
<option value="">-</option>
<?php while ($row = mysql_fetch_assoc($res_cat)) { ?>        
<option value='<?php echo $row['id']; ?>'><?php echo $row['name']; ?></option>              
<?php } ?>
</select>