Simple Blockchain in PHP

index.php

<?php

// This is the blockchain example from The Imposter's Handbook Season 2 book.

class Block
{
    public static function mine(array $transactions, float $timestamp, int $difficulty, ?Block $previous = null)
    {
        if ($difficulty < 2) throw new \Exception("This isn't much of a challenge");
        if ($difficulty > 18) throw new \Exception("Do polar icecaps matter to you?");

        $lookingFor = str_repeat("0", $difficulty);
        $mined = false;

        $start = microtime(true);
        $nonce = 0;

        do {
            $possibleHash = static::hashData($transactions, $timestamp, $nonce, $previous);

            $mined = str_starts_with($possibleHash, $lookingFor);

            if (! $mined) {
                $nonce++;
            } else {
                $end = microtime(true);
                $elapsed = $end - $start;

                echo sprintf("Found it: %s\nThe nonce is: %s\nIt took exactly %s seconds\n\n", $possibleHash, $nonce, $elapsed);

                return new Block(
                    $possibleHash,
                    $transactions,
                    $timestamp,
                    $nonce,
                    $previous,
                );
            }
        } while (! $mined);
    }

    private static function hashData(array $transactions, float $timestamp, int $nonce, ?Block $previous = null)
    {
        return hash('sha256', json_encode([
            'transactions' => $transactions,
            'timestamp' => $timestamp,
            'nonce' => $nonce,
            'previous' => $previous,
        ]));
    }

    private function __construct(
        private string $key,
        private array $transactions,
        private float $timestamp,
        private int $nonce,
        private ?Block $previous = null,
    ) {
    }

    public function isValid()
    {
        return $this->key === static::hashData($this->transactions, $this->timestamp, $this->nonce, $this->previous);
    }

    public function getKey(): string
    {
        return $this->key;
    }

    public function getPrevious(): ?Block
    {
        return $this->previous;
    }
}

class BlockChain
{
    public function __construct(
        private array $blocks = [],
        private ?Block $head = null,
    ) {}

    public function createBlock($transactions, $timestamp)
    {
        $block = Block::mine($transactions, $timestamp, $difficulty = 5, $this->head);

        $this->blocks[$block->getKey()] = $this->head = $block;
    }

    public function traverse($callback)
    {
        $head = $this->head;

        while ($head) {
            $callback($head);

            $head = $head->getPrevious();
        }
    }
}

// Executing it...

$blockchain = new BlockChain();

$transaction1 = [
    ['from' => 'me', 'to' => 'you', 'amount' => 1000],
    ['from' => 'you', 'to' => 'me', 'amount' => 500],
];

$transaction2 = [
    ['from' => 'me', 'to' => 'you', 'amount' => 600],
    ['from' => 'you', 'to' => 'me', 'amount' => 1200],
];

$transaction3 = [
    ['from' => 'me', 'to' => 'you', 'amount' => 300],
    ['from' => 'you', 'to' => 'me', 'amount' => 1300],
];

$blockchain->createBlock($transaction1, microtime(true));
$blockchain->createBlock($transaction2, microtime(true));
$blockchain->createBlock($transaction3, microtime(true));

echo "Traversing the blocks...\n\n";

$blockchain->traverse(function ($block) {
    echo "Block: {$block->getKey()}\n\n";
});