分类树方法

/**
 +----------------------------------------------------------
 *  分类树
 +----------------------------------------------------------
 */
function list_to_tree($list, $pk='id',$pid = 'pid',$child = '_child',$root=0)
{
	// 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 = $data[$pid];
			if ($root == $parentId) {
				$tree[] =& $list[$key];
			}else{
				if (isset($refer[$parentId])) {
					$parent =& $refer[$parentId];
					$parent[$child][] =& $list[$key];
				}
			}
		}
	}
	return $tree;
}