php - Can anyone clearly explain this infinite hierarchical function?

Question

I can use it, but I can’t understand it every time I look at it. Can anyone explain and clear it up? &How to understand reference assignment here?

$items = array(
    1 => array('id' => 1, 'pid' => 0, 'name' => '安徽省'),
    2 => array('id' => 2, 'pid' => 0, 'name' => '浙江省'),
    3 => array('id' => 3, 'pid' => 1, 'name' => '合肥市'),
    4 => array('id' => 4, 'pid' => 3, 'name' => '长丰县'),
    5 => array('id' => 5, 'pid' => 1, 'name' => '安庆市'),
);
function getTree($items){
    $tree = array();
    foreach($items as $item){
        if(isset($items[$item['pid']])){
            $items[$item['pid']]['son'][] = &$items[$item['id']];
        }else{
            $tree[] = &$items[$item['id']];
        }
    }
    return $tree;
}

Answer 1

Mainly the use of references, which can simplify the test:

• When quoted:

$item1= array('id' => 1, 'pid' => 0, 'name' => '安徽省');
$item2= array('id' => 2, 'pid' => 0, 'name' => '浙江省');
$item3=array('id' => 3, 'pid' => 1, 'name' => '合肥市');
$tree[] = &$item1;//注意引用
$item1['son']=&$item2;//注意引用
$item2['son']=$item3;
print_r($tree);

• Without citation:

$item1= array('id' => 1, 'pid' => 0, 'name' => '安徽省');
$item2= array('id' => 2, 'pid' => 0, 'name' => '浙江省');
$item3=array('id' => 3, 'pid' => 1, 'name' => '合肥市');
$tree[] = $item1;//注意无引用
$item1['son']=$item2;//注意无引用
$item2['son']=$item3;
print_r($tree);

When there is a reference, changes to the sub-elements will be displayed in the entire array, but when there is no reference, changes to the sub-elements will have no impact on the entire array.

Answer 2

Is there no one here? Please explain

Answer 3

is like this, items in the brackets of foreach => a and items => b in the loop body exist in two places in memory. After using &, b will point to a i.e. the real items.

But after php7, foreach has changed a bit => php7 foreach is not backward compatible

Answer 4

Reference assignment means that the left side of the assignment points directly to the area in the memory where the value is stored, instead of opening up a new space to receive a copy of the data.

So, in the parent-child level relationship, reference assignment directly points the ['son'] in the parent element directly to the storage area of ​​the child element, instead of just storing the value. Each parent element points ['son'] to the storage area of ​​the corresponding child element. In this way, it has actually been connected to form a tree structure in the memory. Since all parent elements have ['son'] all point to the memory area of the child element , so the parent-child relationship in the output array is basically the same as the data relationship tree in the memory.

Answer 5

Reference @vishun

$items = array(
    1 => array('id' => 1, 'pid' => 0, 'name' => '安徽省'),
    2 => array('id' => 2, 'pid' => 0, 'name' => '浙江省'),
    3 => array('id' => 3, 'pid' => 1, 'name' => '合肥市'),
    4 => array('id' => 4, 'pid' => 3, 'name' => '长丰县'),
    5 => array('id' => 5, 'pid' => 1, 'name' => '安庆市'),
);

---

//&引用赋值在这儿如何理解？
$items[1]['son'][] = &$items[3];
$items[3]['son'][] = &$items[2];
print_r($items[1]);
//简化了结果，可以直接看son
//Array
//(
//    [id] => 1,[pid] => 0,[name] => 安徽省,
//    [son] => Array(
//        [0] => Array(
//            [id] => 3,[pid] => 1,[name] => 合肥市,
//            [son] => Array(
//                [0] => Array(
//                    [id] => 2,[pid] => 0,[name] => 浙江省
//                )
//            )
//        )
//    )
//)

---

//无限分级函数么？
function getTree($items){
    $tree = array();
    foreach($items as $item){
        if(isset($items[$item['pid']])){//关键是看这个判断，是顶级分组就给$tree，不是的话继续拼凑子分组（结合上述&用法）
            $items[$item['pid']]['son'][] = &$items[$item['id']];

        }else{
            $tree[] = &$items[$item['id']];
        }
    }
    return $tree;
}

Answer 6

After seeing the above answers, I still want to answer. The use of references in PHP is similar to the usage of pointers in C/C++. It is equivalent to operating the pointer of this variable. In this case, operating the reference variable in the function will also trigger The variable itself changes.
References can bring some benefits: because they operate "pointers" directly, they are very efficient and will not cause unnecessary waste of memory and consume the performance of opening up memory;