function category_tree($data){
$temp = $rst = array();
foreach($data as $v) $temp[$v['id']] = $v;
foreach($temp as $k=>$v){
if(isset($temp[$v['pid']])){
$temp[$v['pid']]['child'][] = &$temp[$k];
}else{
$rst[] = &$temp[$k];
}
}
return $rst;
}