$tmpMap[$item[$pid]][$son][] 无限分类这样定义报错 [ 1.0 版本 ]

 function gen_tree($items, $id = 'id', $pid = 'pid', $son = 's') {
		$tree = array();
		$tmpMap = array();
	
		foreach ($items as $item) {
			$tmpMap[$item[$id]] = $item;
		}
		foreach ($items as $key => $item) {
			if (isset($tmpMap[$item[$pid]])) {

				$tmpMap[$item[$pid]][$son][] = &$tmpMap[$item[$id]];
				
			} else {
				$tree[] = &$tmpMap[$item[$id]];
			}
		}
		//print_r(CJSON::encode($tmpMap));
		unset($tmpMap);
		
		return $tree;
	}

报错：Indirect modification of overloaded element of Menu has no effect