PHP/JS:动态Dropboxes和回显SELECTED值

我目前使用的是动态下拉菜单（可靠的选择框）。我直接从MySQL数据库中提取值（如果你好奇的话，这里是我如何设置数据库的）。我能够获得每个表的值并相应地显示它们。我遇到的问题是回显每个selct框的SELECTED值。我创建了一个JS函数，它将请求postfile.php，然后它将回显每个框的SELECTED值。我没有得到任何回应。我已经和firebug核实过了，但没有任何消息发布。

我怎样才能做到这一点？还是我处理这件事错了？或者有更好的方法吗？示例

使用HTML/PHP

<?php
include ('includes/dbConnect.php');
try {
    $pdo = get_database_connection();   
    $sql = "SELECT * 
            FROM `categories`
            WHERE `master_id` = 0";
    $statement = $pdo->query($sql);
    $list = $statement->fetchAll(PDO::FETCH_ASSOC);

} catch(PDOException $e) {
    echo 'There was a problem';
} 
?>
    <select name="main" id="main" size="7" class="update">
        <option value="">Select one</option>
        <?php if (!empty($list)) { ?>
        <?php foreach($list as $row) { ?>
        <option value="<?php echo $row['id']; ?>">
            <?php echo $row['name']; ?>
        </option>
        <?php } ?>
        <?php } ?>
    </select>
    <select name="subc1" id="subc1" size="7" class="update" disabled="disabled" hidden="hidden">
        <option value="">----</option>
    </select>
    <select name="subc2" id="subc2" size="7" class="update" disabled="disabled" hidden="hidden">
        <option value="">----</option>
    </select>
    <select name="subc3" id="subc3" size="7" class="update" disabled="disabled" hidden="hidden">
        <option value="">----</option>
    </select>

JS

 <script type="text/javascript">
        $(document).ready(function () {
            $('#main).change(function() {
                  if ($(this).val()!='
            ') {
                    $("#subc1").load("postfile.php",{main_id: $(this).val()});
                    //$("#subc1").removeAttr('
            disabled hidden ');
                  }
                });
                //code on change of sel_source
                $('#subc1 ').change(function() {
                  if ($(this).val()!='
            ') {
                    $("#subc2").load("postfile.php",{subc1_id: $(this).val()});
                    //$("#colour").removeAttr('
            disabled ');
                  }
                });
                $('#subc2 ').change(function() {
                  if ($(this).val()!='
            ') {
                    $("#subc3").load("postfile.php",{subc2_id: $(this).val()});
                    //$("#colour").removeAttr('
            disabled ');
                  }
                });
            });
    </script>

PHP-postfile.PHP

 if(isset($_REQUEST['main_id']) && !empty($_REQUEST['main_id'])) {
    try {
    include ('../includes/dbConnect.php');
        $pdo = get_database_connection();

        $sql = ("select * from `categories` where id='".$_REQUEST['main_id']."' ");
        $result = $con->prepare($sql); 
        $result->execute(); 
        $number_of_rows = $result->fetchColumn();
    }catch(PDOException $e) {
        echo 'There was a problem';
    } 
        if($number_of_rows > 0) {
            $output = '<option value="">Select</option>';
            while($row = mysql_fetch_assoc($result)) {              
                    $output .= '<option value="'.$row['id'].'">'.$row['name'].'</option>';
            }
        } else {
            $output = '<option value="">Select</option>';
        }
        echo $output;
    } 
    if(isset($_REQUEST['subc1_id']) && !empty($_REQUEST['subc1_id'])) {
        $result = mysql_query("select * from table where id='".$_REQUEST['subc1_id']."' ");
        if($number_of_rows > 0) {
            $output = '<option value="">Select</option>';
            while($row = mysql_fetch_assoc($result)) {              
                    $output .= '<option value="'.$row['id'].'">'.$row['name'].'</option>';
            }
        } else {
            $output = '<option value="">Select</option>';
        }
        echo $output;
    } 
    if(isset($_REQUEST['subc2_id']) && !empty($_REQUEST['subc2_id'])) {
        $result = mysql_query("select * from table where id='".$_REQUEST['subc2_id']."' ");
        if($number_of_rows > 0) {
            $output = '<option value="">Select</option>';
            while($row = mysql_fetch_assoc($result)) {              
                    $output .= '<option value="'.$row['id'].'">'.$row['name'].'</option>';
            }
        } else {
            $output = '<option value="">Select</option>';
        }
        echo $output;
    }