How does PHP achieve unlimited classification through the idea of ​​passing references (simple code), php reference_PHP tutorial

How does PHP achieve infinite classification (simple code) through the idea of ​​passing references? PHP references

unlimited classification, mainly by storing the id and classification path of the upper-level classification

In my Simpla, infinite classification is used, and PHP's passing idea is used to achieve infinite classification, which can perfectly display classification patterns like this.

id pid name
1 0   Sichuan
2 0   Chongqing
3 1 Chengdu
4 1 Mianyang
5 3 High-tech Zone

The code is as follows:

/**
  * 数组变成无限级分类--传引用思想
  * @param array $items
  * @return array
  */
 public static function get_tree($orig) {
  //解决下标不是1开始的问题
  $items = array();
  foreach ($orig as $key => $value) {
   $items[$value[‘id‘]] = $value;
  }
  //开始组装
  $tree = array();
  foreach ($items as $key => $item) {
   if ($item[‘pid‘] == 0) { //为0，则为1级分类
    $tree[] = &$items[$key];
   } else {
    if (isset($items[$item[‘pid‘]])) { //存在值则为二级分类
     $items[$item[‘pid‘]][‘child‘][] = &$items[$key]; //传引用直接赋值与改变
    } else { //至少三级分类
     //由于是传引用思想，这里将不会有值
     $tree[] = &$items[$key];
    }
   }
  }
  return $tree;
 }

The above content is very simple. If there are any mistakes or better methods, I hope we can communicate with each other. Thanks. !