同一按钮，实现升序，降序 排序

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");
			}