<?php 

declare(strict_types=1);

class Node {
    function __construct(
        public $data,
        public ?Node $next = null,
    ) {}
}

class LinkedList { 
    protected $head; 
}

interface ISinglyLinkedList {
    function push($data);
}

class SinglyLinkedList extends LinkedList implements ISinglyLinkedList {
    function push($data) {
        $node = new Node($data);
        $node->next = $this->head;
        $this->head = $node;
    }
}

function main() {
    $list = new SinglyLinkedList();
    for ($i=5; $i>0; $i--) { $list->push($i); }
    print_r($list);  
}; main();