将有层级关系的php数组转换为层级数组

level.php

<?php
//需要数据的键与子项目id相同
$arr = [
  1 => [
    "id" => 1,
    "pid" => 0,
    "value" => 1
  ],
  2 => [
    "id" => 2,
    "pid" => 0,
    "value" => 2
  ],
  3 => [
    "id" => 3,
    "pid" => 1,
    "value" => 1.1
  ],
  4 => [
    "id" => 4,
    "pid" => 2,
    "value" => 2.1
  ],
];
$levelArray = [];
foreach($arr as $key => $value){
  if($value['pid'] !== 0){
    $arr[$value['pid']]['sub'][] = &$arr[$key];
  }else{
    $levelArray[] = &$arr[$key];
  }
}