php - How to realize three-level classification display output?

Question

数据库文件xd_item：
id    pid     item         lead     ...
1       0     标题名称一  
2       0     标题名称二
3       1     中标题名称1-1
4       1     中标题名称1-2
5       2     中标题名称2-1
6       2     中标题名称2-2
8       3     小标题名称1-1-1
9       6     小标题名称2-2-1
10      6     小标题名称2-2-2
11      2     中标题名称2-3

---------------------
输出效果：
序号     项目名称
1       标题名称一      
1.1     中标题名称1-1
1.1.1   小标题名称1-1-1
1.2     中标题名称1-2
2       标题名称二
2.1     中标题名称2-1
2.2     中标题名称2-2
2.2.1   小标题名称2-2-1
2.2.2   小标题名称2-2-2
2.3     中标题名称2-3
============================
要在PHP实现这个输出效果，要怎么实现呢？ 

Answer 1

First query those whose pid is 0, traverse those whose pid is 0, query those whose pid is the id of these items, and then output in a loop

Answer 2

If it is Oracle, you can use level() and sys_connect_by_path() functions to implement it.
If you do not use functions, you can build a view to implement it.

Answer 3

This is a reference to my method, which uses recursion to traverse. As long as it deals with data structures, there are methods, it just depends on the efficiency

static public function toLevel($cate, $delimiter = '|——', $parent_id = 0, $level = 0) {

    $arr = array();
    foreach ($cate as $v) {
        if ($v['parent_id'] == $parent_id) {
            $v['type'] = $level + 1;
            $v['delimiter'] = str_repeat($delimiter, $level);
            $arr[] = $v;
            $arr = array_merge($arr, self::toLevel($cate, $delimiter, $v['cate_id'], $v['type']));
        }
    }

    return $arr;

}

Answer 4

Query the data using function calls. The first one perfectly supports your needs, and the second one is recursive storage of data

$arr = [
    0=>['id'=>1,'pid'=>0,'title'=>'标题名称一'],
    1=>['id'=>2,'pid'=>0,'title'=>'标题名称一'],
    2=>['id'=>3,'pid'=>1,'title'=>'标题名称一'],
     3=>['id'=>4,'pid'=>1,'title'=>'标题名称一'],
    4=>['id'=>5,'pid'=>2,'title'=>'标题名称一'],
    5=>['id'=>6,'pid'=>2,'title'=>'标题名称一'],
    6=>['id'=>7,'pid'=>3,'title'=>'标题名称一'],
    7=>['id'=>8,'pid'=>3,'title'=>'标题名称一'],
    8=>['id'=>9,'pid'=>6,'title'=>'标题名称一'],
    9=>['id'=>10,'pid'=>6,'title'=>'标题名称一'],
    10=>['id'=>11,'pid'=>2,'title'=>'标题名称一'],
];
$result = foreachd($arr,0);var_dump($result);
function foreachd($arr,$pid,$showpage = '') {
    $setpage = 1;
    $result = array();
    foreach($arr as $key=>$val) {
        if($val['pid'] == $pid) {
            $setshowpage = $showpage == '' ? $setpage : $showpage.'.'.$setpage;
            $arr[$key]['page'] = $setshowpage;
            $setpage++;
            $setarray = ['page'=>$setshowpage,'title'=>$val['title']];
            $result[] = $setarray;
            $result = array_merge($result,foreachd($arr,$val['id'],$setshowpage));
            
        }
    }
    return $result;
}
/*function foreachd($arr,$pid) {
    $return = array();
    foreach($arr as $val) {
        if($val['pid'] == $pid) {
            $return[$val['id']]['title'] = $val['title'];
            $childrendata = foreachd($arr,$val['id']);
            if($childrendata) {
                $return[$val['id']]['children'] = $childrendata;
            }
            
        }
    }
    return $return;
}*/