PHP把从数据库查出的数据，根据主键外键，变成关系树(多叉树)，从而转成json给前端使用。【递归思想，原创】

$arr = [
    ['id' => 3, 'pid' => 0, 'name' => 'A'],
    ['id' => 4, 'pid' => 3, 'name' => 'B'],
    ['id' => 5, 'pid' => 3, 'name' => 'C'],
    ['id' => 6, 'pid' => 3, 'name' => 'D'],
    ['id' => 7, 'pid' => 5, 'name' => 'E'],
    ['id' => 8, 'pid' => 4, 'name' => 'F'],
    ['id' => 9, 'pid' => 6, 'name' => 'G'],
    ['id' => 10, 'pid' => 0, 'name' => 'H'],
    ['id' => 11, 'pid' => 9, 'name' => 'I'],
    ['id' => 12, 'pid' => 8, 'name' => 'J'],
    ['id' => 13, 'pid' => 6, 'name' => 'K'],
    ['id' => 14, 'pid' => 10, 'name' => 'L'],
    ['id' => 15, 'pid' => 0, 'name' => 'M'],
    ['id' => 16, 'pid' => 4, 'name' => 'N'],
];


/**
 * @param $arr  传入的数组
 * @param $pid  连接关系参照字段
 * @param $pKey 最终生成关系组的下标
 * @param $tree 返回最终的处理结果 
 * @return array
 */
function ToTree($arr,$pid,$pKey,&$tree){
    $currentDeep = [];
    foreach($arr as $key => $value){
         if($value['pid'] == $pid) { # 有儿子
             $value[$pKey] = ToTree($arr,$value['id'],$pKey,$tree); # 找儿子
             $currentDeep[] =  $value;
         }
    }
    if($currentDeep && $pid == 0){ # 当前根节点的儿子都找完了, 把当前根节点的儿子全部归位
        $tree = $currentDeep;
    }
    return $currentDeep;
}

$tree = [];
ToTree($arr,0,'child',$tree);
#echo json_encode($tree);
print_r($tree);