无限极分类，递归分类

有一组数组如下：

<?php
$items = array(
    1 => array('id' => 1, 'pid' => 0, 'name' => '江西省'),
    2 => array('id' => 2, 'pid' => 0, 'name' => '黑龙江省'),
    3 => array('id' => 3, 'pid' => 1, 'name' => '南昌市'),
    4 => array('id' => 4, 'pid' => 2, 'name' => '哈尔滨市'),
    5 => array('id' => 5, 'pid' => 2, 'name' => '鸡西市'),
    6 => array('id' => 6, 'pid' => 4, 'name' => '香坊区'),
    7 => array('id' => 7, 'pid' => 4, 'name' => '南岗区'),
    8 => array('id' => 8, 'pid' => 6, 'name' => '和兴路'),
    9 => array('id' => 9, 'pid' => 7, 'name' => '西大直街'),
    10 => array('id' => 10, 'pid' => 8, 'name' => '东北林业大学'),
    11 => array('id' => 11, 'pid' => 9, 'name' => '哈尔滨工业大学'),
    12 => array('id' => 12, 'pid' => 8, 'name' => '哈尔滨师范大学'),
    13 => array('id' => 13, 'pid' => 1, 'name' => '赣州市'),
    14 => array('id' => 14, 'pid' => 13, 'name' => '赣县'),
    15 => array('id' => 15, 'pid' => 13, 'name' => '于都县'),
    16 => array('id' => 16, 'pid' => 14, 'name' => '茅店镇'),
    17 => array('id' => 17, 'pid' => 14, 'name' => '大田乡'),
    18 => array('id' => 18, 'pid' => 16, 'name' => '义源村'),
    19 => array('id' => 19, 'pid' => 16, 'name' => '上坝村'),
);?>

常见递归方法：

<?php
function getTree($arr,$parent_id){
    $tree = [];
    foreach ($arr as $k=>$v){
        if ($v['pid'] == $parent_id){
            $v['son'] = $this->getTree($arr,$v['id']);
            if ($v['son'] == null){
                unset($v['son']);
            }
            $tree[] = $v;
        }
    }
    return $tree;
}
?>

某开源网站方法：

A方法：

<?php
function genTree5($items) {
    foreach ($items as $item)
        $items[$item['pid']]['son'][$item['id']] = &$items[$item['id']];
    return isset($items[0]['son']) ? $items[0]['son'] : array();
}?>

B方法：

<?php/**
 * 将数据格式化成树形结构
 * @author Xuefen.Tong
 * @param array $items
 * @return array
 */
function genTree9($items) {
    $tree = array(); //格式化好的树
    foreach ($items as $item)
        if (isset($items[$item['pid']]))
            $items[$item['pid']]['son'][] = &$items[$item['id']];
        else
            $tree[] = &$items[$item['id']];
    return $tree;
}?>