ช่วยดูโค้ดทีครับจะทำ listbox แบบ optgroup แต่ผลลัพธ์ไม่ออก

<?php 
error_reporting(E_ALL ^ E_NOTICE);
    $host = "localhost";
    $user = "root"; 
    $passwd = "root";
    $db = "ajaxdemo";

    $connect = mysql_connect($host,$user,$passwd) ;

    mysql_select_db($db);
    mysql_query('SET NAMES utf8');

    $sql = "SELECT job FROM user ORDER BY id, job";
                     $result = mysql_query($sql) or die(mysql_error());
                     $prev='';
                     while ($row = mysql_fetch_array($result)) { 
                     if ($prev != $row['FirstName']) {
                         if ($prev) {
                         echo "<optgroup>";
                         echo "<optgroup label='--------------'>";
                         $prev = $row['id'];
                         }
                     }
                         echo "<option value='".$row['id']."'>".$row['job']."</option>";                           
                         echo "</optgroup>";
                     }                                  
?>