function list_to_tree($list, $root = 0,$pk = 'id', $pid = 'pid', $child = '_child') {	// 创建Tree	$tree = array();	if (is_array($list)){		// 创建基于主键的数组引用		$refer = array();		foreach ($list as $key => $data) {			$refer[$data[$pk]] = &$list[$key];		} 		foreach ($list as $key => $data) {			// 判断是否存在parent			$parentId=0;			if(isset($data[$pid])){				$parentId =$data[$pid] ;			}			if ($root == $parentId) {				$tree[] = &$list[$key];			} else {				if (isset($refer[$parentId])) {					$parent = &$refer[$parentId];					$parent[$child][] = &$list[$key];				} 			}		} 	}	return $tree;}

function tree_to_list($tree, $level = 0,$pk = 'id', $pid = 'pid', $child = '_child'){	$list = array();	if (is_array($tree)){		foreach($tree as $val) {			$val['level']=$level;			$child=$val['_child'];			if(isset($child)){				if (is_array($child)){					unset($val['_child']);					$list=$val;					$list = array_merge($list, tree_to_list($child,$level+1));				}			}else{				$list=$val;			}		}		return $list;	}}