ThinkPHP组合查询AND和OR一起用

如下示例：

$_where 和 $where组合查询

$_where之间用OR

$where之间用AND

$condition = I('post.condition');
switch($condition){
	case 1:
		$where['t.digest'] = array('gt',0);
		break;
	case 2:
		$where['t.displayorder'] = 4;
		break;
	case 3:
		$where['t.type'] = 1;
		break;
	default:
		$_where['t.type']		= 1;
		$_where['t.displayorder']= 4;
		$_where['t.digest']		= array('gt',0);
		$_where['_logic']		= 'or';
		$where['_complex']		= $_where;
}
// 板块筛选
if($_POST['fid']){
	$where['t.fid'] = I('post.fid');
}
// 时间筛选
if($_POST['start_time'] and $_POST['end_time']){
	$where['t.add_time'] = array(
		array('egt',strtotime(I('post.start_time'))),
		array('elt',strtotime(I('post.end_time')))
	);
}else{
	if($_POST['start_time']){
		$where['t.add_time'] = array('egt',strtotime(I('post.start_time')));
	}else if($_POST['end_time']){
		$where['t.add_time'] = array('elt',strtotime(I('post.end_time')));
	}
}