Simple iterable linked list storing keyvalues. No removal implemented for simplicity. Easy & fast removal could be implemented with a Double Linked List approach. Requires PHP 7.4 (typed properties)

SimpleLinkedList.php

<?php
class LinkedNode {
    protected LinkedNode $next;
    protected $value;
    protected string $key;
    public function __construct(string $key, $value) {
        $this->value = $value;
        $this->key = $key;
    }
    public function key(): string {
        return $this->key;
    }
    public function value() {
        return $this->value;
    }
    public function next(): ?LinkedNode {
        return $this->next ?? null;
    }
    public function add(LinkedNode $node): LinkedNode {
        $this->next = $node;
        return $this;
    }
}

class LinkedList implements Iterator {
    protected LinkedNode $firstNode;
    protected LinkedNode $lastNode;
    protected ?LinkedNode $iteratorCurrentNode;
    public function dump() {
        foreach($this as $node) {
            echo "[" . $node->key().":".$node->value() . "] -> ";
        }
        echo "END".PHP_EOL;
    }
    public function dumpKeys() {
        foreach($this as $node) {
            echo "[" . $node->key()."] -> ";
        }
        echo "END".PHP_EOL;
    }
    public function get(string $key): ?LinkedNode {
        foreach($this as $node) {
            if ($node->key()===$key) return $node;
        }
        return null;
    }
    public function add(LinkedNode $node): LinkedList {
        if (isset($this->lastNode)) $this->lastNode->add($node);
        $this->lastNode = $node;
        if (!isset($this->firstNode)) $this->firstNode = $node;
        if (!isset($this->iteratorCurrentNode)) $this->iteratorCurrentNode = $this->firstNode;
        return $this;
    }
    public function next() {
        if (isset($this->iteratorCurrentNode) && $this->iteratorCurrentNode->next()) {
            $this->iteratorCurrentNode = $this->iteratorCurrentNode->next();
        } else {
            $this->iteratorCurrentNode = null;
        }
    }
    public function current(): ?LinkedNode {
        return $this->iteratorCurrentNode;
    }
    public function rewind() {
        $this->iteratorCurrentNode = $this->firstNode;
    }
    public function key(): ?string {
         return $this->iteratorCurrentNode->key() ?? null;
    }
    public function valid() {
        return $this->iteratorCurrentNode!=null;
    }
}

$linkedList = new LinkedList();
$linkedList->add(new LinkedNode("node-1", "Node value 1"));
$linkedList->add(new LinkedNode("node-2", "Node value 2"));
$linkedList->add(new LinkedNode("node-3", "Node value 3"));
$linkedList->dumpKeys();

echo "Value of node-2: ". ($linkedList->get("node-2")->value() ?? null) . PHP_EOL;

// EXPECTED OUTPUT ---------------
// [node-1] -> [node-2] -> [node-3] -> END
// Value of node-2: Node value 2