php - 有人可以解释这 2 个引用用法在 PHP 中的区别吗？

Question

下面的代码说明了 PHP 引用的奇怪行为:

<?php

function this_works()
{
    $root = array('name'=>'root', 'children'=>array());
    $level_1 = array('name'=>'level_1', 'children'=>array());
    $item1 = array('name'=>'level_2_1', 'children'=>array());
    $item2 = array('name'=>'level_2_2', 'children'=>array());

    $croot = &$root;

    $croot['children'][] = &$level_1;

    $croot = &$level_1;

    $croot['children'][] = &$item1;
    $croot['children'][] = &$item2;

    $croot = &$root;

    print_r($croot);
}    

function this_fails()
{    
    $root = array('name'=>'root', 'children'=>array());
    $level_1 = array('name'=>'level_1', 'children'=>array());
    $item1 = array('name'=>'level_2_1', 'children'=>array());
    $item2 = array('name'=>'level_2_2', 'children'=>array());
    $croot = &$root;

    $stack = array();

    $croot['children'][] = &$level_1;
    $crootref = &$croot;

    array_push($stack, $crootref);

    $croot = &$level_1;

    $croot['children'][] = &$item1;
    $croot['children'][] = &$item2;

    # this works, assignment below - doesn't... WHY?
    #$x = array_pop($stack);
    #var_dump($x);

    $croot = array_pop($stack);

    print_r($croot);
}    

this_works();
echo "------------------\n";
this_fails();

?>

第一个函数提供了预期的结果，而第二个失败并声称递归循环:

Array
(
    [name] => root
    [children] => Array
        (
            [0] => Array
                (
                    [name] => level_1
                    [children] => Array
                        (
                            [0] => Array
                                (
                                    [name] => level_2_1
                                    [children] => Array
                                    (
                                    )

                                )

                            [1] => Array
                                (
                                    [name] => level_2_2
                                    [children] => Array
                                        (
                                        )

                                )

                           )

                )

        )

)
------------------
Array
(
    [name] => root
    [children] => Array
        (
            [0] => Array
                (
                    [name] => root
                    [children] => Array
 *RECURSION*
                )

    )

)

奇怪的是，如果在第二个函数中，中间变量将用于从堆栈中获取值，结果再次正常。我不明白这是怎么回事。我如何获得根元素作为由于一次攻击，多次成为自己的 child ？

最初，我需要从 XML 构建树(使用 sax 解析器)并打算让“当前根”指向树节点当前级别并将其插入/弹出堆栈并向其添加子元素，但是，令人惊讶的是，由于所展示的问题，我未能实现该方案通过以上两个函数。

那么，这种做法有什么问题呢？

Answer 1

这是因为 PHP 在数组内部引用的方式很奇怪。在右侧使用引用进行正常(不是通过引用)赋值通常不会将左侧变成引用，但数组内部的引用会保留在赋值中，即使没有引用运算符也是如此。

我在下面评论了您的代码以帮助解释正在发生的事情，我希望我已经正确解释了这一点，现在是凌晨 1 点，我有点累了。

$root = array('name'=>'root', 'children'=>array());
$level_1 = array('name'=>'level_1', 'children'=>array());
// $croot and $root now point to the same variable
$croot = &$root;

$stack = array();

$croot['children'][] = &$level_1;
// $crootref, $croot and $root all now point to the same variable
$crootref = &$croot;
// $stack[0], $crootref, $croot and $root all now point to the same variable.
// $stack[0]['children'][0], $level_1, $croot['children'][0] point to the same variable
array_push($stack, $crootref);
// $croot, $level_1 and $stack[0]['children'][0] now point to the same variable
// Infinite loop is caused as $stack[0]['children'][0] is now an alias for $croot
// which contains $croot['children'][0] which is an alias for $stack[0]['children'][0]
// which is an alias for $croot which contains....
$croot = &$level_1;