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>

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");
}