Código PHP:
<select name="username">
<option selected disabled>-- Selecciona una usuario --</option>
<?
$connect = mysql_query("SELECT * FROM ac_users ORDER BY username ASC");
while ($row = mysql_fetch_array($connect)) {
$username = $row['username'];
?>
<option value="<?=$username?>"><?=$username?></option>
<?
}
?>
</select>