Why does it happen when assigning a value to a property of an object in it?

Question

PHP

I was learning php and came across one example.
Why if you assign an array property to an object , then it is inside the $test variable.
If it 's a normal property , then it's outside the $test variable.

<?php
class Foo {
    public $test = [];
  
    public function & __get($name) {
        return $this->test[$name];
    }
  
  public function out() {
        return $this;
    }
}

$foo = new Foo();


$foo->arr[1] = 'one';
$foo->arr[2] = 'two';
$foo->str = 'three';

echo '<pre>';
print_r($foo->out());
print_r($foo->arr);

Here is the result:

print_r output

Foo Object
(
    [test] => Array
        (
            [arr] => Array
                (
                    [1] => one
                    [2] => two
                )

        )

    [str] => three
)
Array
(
    [1] => one
    [2] => two
)

Var_Dump Output

object(Foo)#1 (2) {
  ["test"]=>
  array(1) {
    ["arr"]=>
    array(2) {
      [1]=>
      string(3) "one"
      [2]=>
      string(3) "two"
    }
  }
  ["str"]=>
  string(5) "three"
}
array(2) {
  [1]=>
  string(3) "one"
  [2]=>
  string(3) "two"
}

And another question about the link & __get() , I know how the link works in variables, but it's not clear in functions

Answer 1

php.net/references.return
First, I'll describe the first part of this expression - $foo->arr:
You are accessing a property arr, but it doesn't exist, so the call is "hooked" by the __get. It accesses the property testusing as the array key the name (arr) given as the parameter. Then it returns you a link ( php.net/references.return ) to the $this->test['arr'];
second part - [1] = 'one':
to access the array element with index 1 of the property obtained earlier by reference. That is, writing $foo->arr[1]after reference resolution is equivalent $this->test['arr'][1]from within the object. Further there is an assignment of a line, it turns out $this->test['arr'][1] = 'one';
Why 'three'out of an array test?
because$foo->str = 'three';here you don't get caught in __get(). This is not getting (get) a property, but setting (set) its value.
PS The
example is really brain-breaking. It is better not to use such things as a return by reference, and even in combination with a magic method.