递归无限分类，帮忙看看代码哪里有问题？

Question

    public $cats = array();    
        
    public function category($fid=0, $level=1, $cats) {
        $sql = "select * from article_cat where cat_fid =:id";
        try {
            $stmt = $this->db->prepare($sql);
            $stmt -> bindParam(":id", $fid, PDO::PARAM_INT);
            $stmt -> execute();
            $results = $stmt->fetchAll(PDO::FETCH_ASSOC);
            foreach ($results as $v) {
                array_push($this->cats, array($v['cat_name'], $level));
                $this-> category($v['cat_id'], $level+1, $this->cats);
                
            }

            return $this->cats;
            
        } catch(Exception $e) {
            die($e->getMessage());
        }
    }

会正确输出所有的分类值
array(6){

[
    0
]=>array(2){
    [
        0
    ]=>string(8)"3G咨询"[
        1
    ]=>int(1)
}[
    1
]=>array(2){
    [
        0
    ]=>string(12)"系统分类"[
        1
    ]=>int(1)
}[
    2
]=>array(2){
    [
        0
    ]=>string(18)"网店帮助分类"[
        1
    ]=>int(2)
}[
    3
]=>array(2){
    [
        0
    ]=>string(12)"新手上路"[
        1
    ]=>int(3)
}[
    4
]=>array(2){
    [
        0
    ]=>string(12)"手机常识"[
        1
    ]=>int(3)
}[
    5
]=>array(2){
    [
        0
    ]=>string(12)"网店信息"[
        1
    ]=>int(2)
}

}
但是如果把代码稍加修改,把cats变量由类属性改成类方法里的一个变量参数（把public $cats = array(); 删除 ），

    public function category($fid=0, $level=1, $cats = array()) {
        $sql = "select * from article_cat where cat_fid =:id";
        try {
            $stmt = $this->db->prepare($sql);
            $stmt -> bindParam(":id", $fid, PDO::PARAM_INT);
            $stmt -> execute();
            $results = $stmt->fetchAll(PDO::FETCH_ASSOC);
            foreach ($results as $v) {
                array_push($cats, array($v['cat_name'], $level));
                $this-> category($v['cat_id'], $level+1, $cats);
                
            }

            return $cats;
            
        } catch(Exception $e) {
            die($e->getMessage());
        }
    }

输出的数据少了，只输出了顶级分类
array(2){

[
    0
]=>array(2){
    [
        0
    ]=>string(8)"3G咨询"[
        1
    ]=>int(1)
}[
    1
]=>array(2){
    [
        0
    ]=>string(12)"系统分类"[
        1
    ]=>int(1)
}

}
为什么？无法理解，问题出在哪里？

Answer 1

先解决下楼主的的问题

楼主的第二段代码核心在于数组变量的引用，由于楼主的代码

public function category($fid = 0, $level = 1,$cats = array())
//$cats由于不是引用变量，所以每次递归，$cats都是临时生成的，所以到最后只有几个值。
//而第一段代码中楼主使用的是类变量保存递归结果，$this->cats在整个类中都是有效的。
//楼主改为一下代码即可

public function category($fid = 0, $level = 1,&$cats = array())

输出结果

Array
(
    [0] => Array
        (
            [0] => 顶级分类1
            [1] => 1
        )

    [1] => Array
        (
            [0] => 1-子级1
            [1] => 2
        )

    [2] => Array
        (
            [0] => 4-子级1
            [1] => 3
        )

    [3] => Array
        (
            [0] => 4-子级2
            [1] => 3
        )

    [4] => Array
        (
            [0] => 1-子级2
            [1] => 2
        )

    [5] => Array
        (
            [0] => 顶级分类2
            [1] => 1
        )

    [6] => Array
        (
            [0] => 2-子级1
            [1] => 2
        )

    [7] => Array
        (
            [0] => 2-子级2
            [1] => 2
        )

    [8] => Array
        (
            [0] => 顶级分类3
            [1] => 1
        )

)

我的解决方式

代码

<?php
$pdo = new PDO("mysql:host=localhost;dbname=test", "root", "root");

function getCategories(PDO $pdo, $pid = 0)
{
    $sql = 'SELECT * FROM `category` WHERE pid=:pid';
    $stmt = $pdo->prepare($sql);
    $stmt->bindParam(':pid', $pid, PDO::PARAM_INT);
    $stmt->execute();
    $data = $stmt->fetchAll(PDO::FETCH_ASSOC);
    foreach ($data as &$row) {
        $row['subs'] = getCategories($pdo, $row['id']);
    }
    return $data;
}

$a = getCategories($pdo);
print_r($a);

数据库

输出结果

Array
(
    [0] => Array
        (
            [id] => 1
            [name] => 顶级分类1
            [pid] => 0
            [subs] => Array
                (
                    [0] => Array
                        (
                            [id] => 4
                            [name] => 1-子级1
                            [pid] => 1
                            [subs] => Array
                                (
                                    [0] => Array
                                        (
                                            [id] => 8
                                            [name] => 4-子级1
                                            [pid] => 4
                                            [subs] => Array
                                                (
                                                )

                                        )

                                    [1] => Array
                                        (
                                            [id] => 9
                                            [name] => 4-子级2
                                            [pid] => 4
                                            [subs] => Array
                                                (
                                                )

                                        )

                                )

                        )

                    [1] => Array
                        (
                            [id] => 5
                            [name] => 1-子级2
                            [pid] => 1
                            [subs] => Array
                                (
                                )

                        )

                )

        )

    [1] => Array
        (
            [id] => 2
            [name] => 顶级分类2
            [pid] => 0
            [subs] => Array
                (
                    [0] => Array
                        (
                            [id] => 6
                            [name] => 2-子级1
                            [pid] => 2
                            [subs] => Array
                                (
                                )

                        )

                    [1] => Array
                        (
                            [id] => 7
                            [name] => 2-子级2
                            [pid] => 2
                            [subs] => Array
                                (
                                )

                        )

                )

        )

    [2] => Array
        (
            [id] => 3
            [name] => 顶级分类3
            [pid] => 0
            [subs] => Array
                (
                )

        )

)

Answer 2

array_push() 函数向第一个参数的数组尾部添加一个或多个元素（入栈），然后返回新数组的长度。
该函数等于多次调用 $array[] = $value。

注释：即使数组中有字符串键名，您添加的元素也始终是数字键。（参见例子 2）
注释：如果用 array_push() 来给数组增加一个单元，还不如用 $array[] =，因为这样没有调用函数的额外负担。
注释：如果第一个参数不是数组，array_push() 将发出一条警告。这和 $var[] 的行为不同，后者会新建一个数组。

array_push(array,value1,value2...)