Build a Singly Linked List

Construct a singly linked list by allocating one node per value and chaining next references. Establishes the node + head + tail model used by every later linked-list lesson.

Algorithm

Canonical input [10, 20, 30, 40] builds the chain head -> 10 -> 20 -> 30 -> 40 -> null with one node appended per step.

Basic Implementation

basic.php

<?php
$values = [10, 20, 30, 40];
$nodes = [];
$head = -1;
$tail = -1;
$i = 0;
while ($i < count($values)) {
	$idx = count($nodes);
	$nodes[] = ["value" => $values[$i], "next_idx" => -1];
	if ($head == -1) {
		$head = $idx;
	} else {
		$nodes[$tail]["next_idx"] = $idx;
	}
	$tail = $idx;
	$i = $i + 1;
}
$cur = $head;
while ($cur != -1) {
	echo $nodes[$cur]["value"] . " -> ";
	$cur = $nodes[$cur]["next_idx"];
}
echo "null\n";

Complexity

Time: O(n) with a tail pointer
Space: O(n) for the chain

Implementation notes

PHP: a tiny associative-array node ["value" => ..., "next_idx" => ...] stored in a plain indexed-array $nodes arena. The integer next_idx field with -1 as the sentinel is the explicit "end-of-list" marker and lets the $head / $tail pointers update the chain without leaning on PHP's SplDoublyLinkedList or an object reference graph.
The replay never shows runtime references; nodes are labelled node(<value>) and the chain view is rendered as 10 -> 20 -> ... -> null.
The arena is collected by PHP's reference-counted GC at scope exit, so the build step stays focused on wiring without an explicit free walk.

node chain Each node carries a `value` and a `next_idx` index into the node arena (`-1` is the end-of-list sentinel).