用例数据

$items = array(
array('id' => 1, 'pid' => 0, 'name' => "江苏"),
array('id' => 2, 'pid' => 0, 'name' => "浙江"),
array('id' => 3, 'pid' => 1, 'name' => "南京"),
array('id' => 4, 'pid' => 1, 'name' => "苏州"),
array('id' => 5, 'pid' => 2, 'name' => "杭州"),
array('id' => 6, 'pid' => 2, 'name' => "宁波"),
);

核心函数

function arrToTree($array, $parentid = 0)
{
    $result = array();
    foreach ($array as $val) {
        if ($val["pid"] == $parentid) {
            $indata = array("id" => $val["id"], "pid" => $parentid, "name" => $val["name"]);
      
            $chidrendata = arrToTree($array, $val["id"]);
            if ($chidrendata) {
                $indata["children"] = $chidrendata;
            }
   
            $result[] = $indata;
        }
    }
    return $result;
}
function treeToList($arr, $parentid = 0)
{
    $array = array();
    foreach ($arr as $val) {
        $indata = array("id" => $val["id"], "pid" => $parentid, "name" => $val["name"]);
        $array[] = $indata;
        if (isset($val["children"])) {
            $children = treeToList($val["children"], $val["id"]);
            if ($children) {
                $array = array_merge($array, $children);
            }
        }
    }
    return $array;
}

使用方法

$result = arrToTree($items);
var_dump($result);

//读取树形结构
foreach ($result as $key => $value) {
	var_dump($value['id'] . "=>" . $value['name']);
	foreach($value['children'] as $v1)
	{
    		var_dump($v1);
	}
}

$list = treeToList($result);
var_dump($list);