<?php

class A
{
    public $key;
    public $parent;
}

class B
{
    public $value;
    public $objects;
}

$a1 = new A();
$a1->key = 'a1';
$a2 = new A();
$a2->key = 'a2';
$a2->parent = $a1;
$a3 = new A();
$a3->key = 'a3';
$a3->parent = $a2;

$b = new B();
$b->objects = [$a1, $a2, $a3];
$b->value = 100;
someFunction($b);

[
'a1' => ['a2' => ['a3' => 100]]
]