前台页面，将order写入，开始时为空
<a href="{:U('find','type=2&keyword='.$keyword.'&catid='.$catid.'&brand_id='.$brand_id.'&user_per='.$user_per.'&order='.$order)}">
<li class="fu_2">
<span class="fu_22">销量</span>
<img src="{:CUSTOM_TEMPLATE_PATH}Index/index/img/faxian_2.png" width="16">
</li>
</a>点按钮给后端php
1第一次点按钮为空，默认给它降序。并给前端升序变量，为第二次点击做准备
2第二次点按钮点升序，提交给PHP。当判断为升序时，将降序变量给前端，为第三次点击做准备
if(!isset($_GET['order'])){
$order = array('sales_volume'=>'desc');
$this->assign('order',"asc");
}elseif(($_GET['order'])=="asc"){
$order = array('sales_volume'=>'asc');
$this->assign('order',"desc");
}else{
$order = array('sales_volume'=>'desc');
$this->assign('order',"asc");
}