<select name="">
<?php
 include 'db.php';
 $getUsers=mysql_query('SELECT id, user_name FROM users');
 if(mysql_num_rows($getUsers))
 {
    echo '<select name="users">';
    while($row=mysql_fetch_array($getUsers))
    {
        echo '<option value="',$row['id'],'">',$row['user_name'],'</option>';
    }
    echo '</select>';
 }
 ?>
</select>