ajax联动

html部分
<div class="field">
    <label class="main">主类别:</label>
    <select name="sh_mc_id" id="sh_mc_id" onchange="get_category();">
        <option value="0">请选择</option>
        <{foreach from = $main item=val}>
        <option value="<{$val['mc_id']}>"><{$val['mc_name']}></option>
        <{/foreach}>
    </select>
    <span class="error"></span>
</div>
<div class="field">
    <label class="main">子类别:</label>
    <span id="cate_son">
        <select name="sh_sc_id" id="sh_sc_id">
            <option value="">请选择</option>
        </select>
    </span>
    <span class="error"></span>
</div>

/**
 *
 * ajax联动
 */
function get_category(){
    var sh_mc_id = $('#sh_mc_id').val();
    var empty_html = '<select name="sh_sc_id" id="sh_sc_id">';
        empty_html+= '<option value="0">请选择</option>';
        empty_html+= '</select>';
    if(sh_mc_id>0){
        var data = {
            'mc_id' : sh_mc_id
        };
        $.ajax({
            type : 'POST',
            url  : '/shop/getCategory',
            data : data,
            dataType : 'json',
            success:function(json_msg){
                if(json_msg.ec == 200) {
                    $('#cate_son').html(json_msg.em);
                } else {

                    $('#cate_son').html(empty_html);
                }
            }
        });
    } else {
        $('#cate_son').html(empty_html);
    }
}

public function getCategoryAction(){
    $mc_id      = $this->request->getIntParam('mc_id');
    $data       = array('ec' => 400, 'em' => 'bad request');
    if ($mc_id > 0) {
        $manager    = new App_Model_Category_MysqlCategorySubCategoryStorage();
        $list       = $manager->getListByMainID($mc_id,0,0);
        $_html      = $this->_deal_show_subCategory($list);
        $data = array(
            'ec' => 200,
            'em' => $_html
        );
    }
    $this->displayJson($data);
}

private function _deal_show_subCategory($list){
    $_html = '<select name="sh_sc_id" id="sh_sc_id">';
    $_html .= '<option value="0">请选择</option>';
    foreach($list as $v){
        $_html .= '<option value="'.$v['sc_id'].'">'.$v['sc_name'].'</option>';
    }
    $_html .= '</select>';
    return $_html;
}