Blockchain Technology

// Simplified version of Blockchain Technology

/**
  * To create digital signature (or hash) of the transaction
  */
class Block{

	public function __construct( $index, $timestamp, $data, $prev_hash = ''){
		$this->index = $index;
		$this->timestamp = $timestamp;
		$this->data = $data;
		$this->prev_hash = $prev_hash;
		$this->hash = $this->calculateHash();
	}

	// calculates the hash of the block
	public function calculateHash(){
		return hash('sha256', $this->index . $this->timestamp . json_encode( $this->data ) );
	}	
}

/**
  * Stores the list of all transaction in an array.
  * Contains the transaction detail of each block in the chain.
  * Also has the hash to check if the data has been tampered.
  */
class Blockchain{
	public static $index = 0;
	public static $chain;
	public function __construct(){
		if( !isset(self::$chain) )
			self::$chain = array( $this->genesisBlock() );
	}

	public function genesisBlock(){
		return new Block(self::$index, time(), 'genesis block: 100', '0');
	}

	public function getLatestBlock(){
		return end( self::$chain );
	}

	public function addBlock( $newBlock ){
		$newBlock->prev_hash = $this->getLatestBlock()->hash;
		$newBlock->hash = $newBlock->calculateHash();
		array_push( self::$chain, $newBlock);
	}

	// checks the integrity of the blockchain
	public function isChainValid(){

		for( $i = 1; $i < count(self::$chain); $i++ ){
			$currentBlock = self::$chain[$i];
			$prevBlock = self::$chain[$i - 1];

			if( $currentBlock->hash !== $currentBlock->calculateHash() ){
				return false;
			}

			if( $currentBlock->prev_hash !== $prevBlock->hash ){
				return false;
			}

		}
		return true;
	}
}

/**
  * Allows to create multiple account.
  * Initiates only single Blockchain.
  * That Blockchain object is used to track all transactions.
  */
class Account{
	// keeps track of unique user account number
	public static $id = 101;
	public static $Blockchain_obj;
	public function __construct( $name, $balance ){
		$this->name = $name;
		$this->id   = self::$id++;
		$this->balance = $balance;
		// makes sure only one instance of Blockchain exists
		if( !isset(self::$Blockchain_obj) )
			self::$Blockchain_obj = new Blockchain();
	}

	public function view_balance(){
		return $this->name . ', your balance is: ' . $this->balance;
	}

	public function transfer( $another_account, $amount ){
		$this->balance -= $amount;
		Blockchain::$index++;
		$data = array( 'To' => $another_account->id, 'amount' => $amount, 'transaction_type' => 'transfer' );
		$block = new Block( Blockchain::$index, time(), $data );
		$another_account->deposit( $amount );
		return self::$Blockchain_obj->addBlock($block);
	}

	public function deposit( $amount ){
		$this->balance += $amount;
	}

	private function withdraw( $amount ){
		$this->balance -= $amount;
	}

}


// create two random accounts
$account1 = new Account( 'John', 200 );
$account2 = new Account( 'Harry', 200 );

// simulate money transfer
$account1->transfer( $account2, 100 );
$account2->transfer( $account1, 50 );

// shows the new balance after transfer
var_dump($account1->view_balance());
var_dump($account2->view_balance());

// shows all the tractions until now
echo print_r( Blockchain::$chain, true );

/** checks if the data has been tinkered
  *	@returns true in this case
  */
var_dump( 'valid chain? ' . Account::$Blockchain_obj->isChainValid() );

// simulate data tampering
var_dump( Blockchain::$chain[1]->data = array( 'amount' => 20000 ) );
var_dump( Blockchain::$chain[1]->calculateHash() );

/** checks whether the chain is valid after tampering
  * returns false cause the data was altered
  */
var_dump( 'valid chain? ' . Account::$Blockchain_obj->isChainValid() );

Output:

string(26) "John, your balance is: 150" 
string(27) "Harry, your balance is: 250"
Array
(
    [0] => Block Object
        (
            [index] => 0
            [timestamp] => 1513702684
            [data] => genesis block: 100
            [prev_hash] => 0
            [hash] => 09fa53c5a5c1b2f9e96a1715b658543612121dd13de3c39eeed761c1e0c39a45
        )

    [1] => Block Object
        (
            [index] => 1
            [timestamp] => 1513702684
            [data] => Array
                (
                    [To] => 102
                    [amount] => 100
                    [transaction_type] => transfer
                )

            [prev_hash] => 09fa53c5a5c1b2f9e96a1715b658543612121dd13de3c39eeed761c1e0c39a45
            [hash] => 03bbb054b83e2a029cf9517975338386e2e2e76050d8e970fc7ad83f30ce4453
        )

    [2] => Block Object
        (
            [index] => 2
            [timestamp] => 1513702684
            [data] => Array
                (
                    [To] => 101
                    [amount] => 50
                    [transaction_type] => transfer
                )

            [prev_hash] => 03bbb054b83e2a029cf9517975338386e2e2e76050d8e970fc7ad83f30ce4453
            [hash] => 846242260d96b36fafccc6c4232e42d975228d7439ff719f3e42b8aca39fac7d
        )

)
string(27) "first check: valid chain? 1"
array(1) { ["amount"]=> int(20000) } 
string(64) "5e4e2cd2a8fbd856e74e4eb45f5db55e07005f36c46c8dbc07babd9a5ae3cc86" 

string(27) "second check: valid chain? "