How does the category tree generation function work?

Question

PHP

Good day!
On the Internet, I found a function for implementing a category tree. But when it was parsed, I did not finally understand how exactly it works.
The implementation of the function itself is here .
I'm interested in exactly how in this piece of code

foreach ($categories as &$category) {
        $map[$category['parent']]['subcategories'][] = &$category;
    }

nested arrays (of different levels of nesting) are formed? I understand that this happens when passing by reference. But how do they appear?
Thanks in advance for your reply!

Answer 1

in the code from the link you provided, the most subtle point is:
is actually the same (short syntax) as

array_push(
    $map[$category['parent']]['subcategories'],
    &$category
);

$category['parent'] is the id of the category. Let's say it's 0 .
That is,a reference to $categories[0 ] will be added to the $map[0]['subcategories'][0] array . If there is another category with the same [parent] , then the link to it will be written in $map[0]['subcategories'][1] . Well, as in the line, it says that the category in the map ( $map ), refers to the category in the list ( $categories ). Then they are set by reference recursively. the interpreter looks in the subcategory - there are links, it looks in the subcategory of links - there are links again, and so on, until the nesting reaches the end.

Answer 2

TO greabock :
It's not entirely clear how this recursion is called.
Here I have made the output of the $map array on each iteration of the loop. Could you explain to me how a nested array arises, for example, in the so-called. section with code name Parent => 2--------------------Category => 6 (line 787 ):

[0] => Array
            (
               [id] => 6
               [parent] => 2
               [name] => Subcategory F
               [subcategories] => Array
               (
               )

            )

Maybe then based on this I can understand the basic principle