Inheritance Example

inheritance.php

<?php

define('DEBUG', true);

function debug($message) {
  if (DEBUG) {
    echo " -D- ", $message, "\n";
  }
}

/**
 * Money Interface
 */
interface IMoney {
  const CURRENCY = null;

  public function __construct($value);
  public function getValue();
  public function setValue($value);
  public function toString();
}

/**
 * Pocket Interface
 */
interface IPocket {
  public function __construct($initialValue, $baseCurrency);
  public function getCurrentState();
  public function addTransaction(Money $value);
  public function listTransactions();
  public function toString();
}

/**
 * Base Money class
 */
abstract class Money implements IMoney {
  protected $value = 0;

  public function __construct($value = 0) {
    $this->value = round((float)$value, 2);
  }

  public function getValue() {
    return $this->value;
  }

  public function setValue($value) {
    $this->value = $value;
  }
}

/**
 * Pocket class
 */
class Pocket {
  protected $value = 0;
  protected $transactions = array();

  public function __construct($initialValue, $baseCurrency = "$") {
    $this->value = round((float)$initialValue, 2);
    $this->baseCurrency = $baseCurrency;
  }
  public function getCurrentState() {
    return $this->value;
  }
  public function addTransaction(Money $value) {
    $this->transactions[] = $value;
    $this->value += $value->getValue() * $this->exchange($value::CURRENCY, $this->baseCurrency);
    debug("Transaction added with value of (" . $value->toString() . ").");
  }
  public function listTransactions() {
    return $this->transactions;
  }
  public function toString($in = "") {
    if ($in == "") {
      $in = $this->baseCurrency;
    }
    $converted = $this->value * $this->exchange($this->baseCurrency, $in);
    if ($in == "HUF") {
      $value = new HungarianForint($converted);
    } else {
      $value = new USDollar($converted);
    }

    return $value->toString() . " with " . count($this->transactions) . " transaction" . (count($this->transactions) > 1 ? "s" : "");
  }
  protected function exchange($from, $to) {
    switch($from . "_" . $to) {
      case "HUF_\$": return 0.0042; break;
      case "\$_HUF": return 237.11; break;
      default: return 1;
    }
  }
}

/**
 * Currencies / Money
 */
class HungarianForint extends Money {
  const CURRENCY = "HUF";

  public function __construct($value = 0) {
    $this->value = (int)$value;
  }

  public function toString() {
    return number_format($this->value, 0, ",", " ") . " " . self::CURRENCY;
  }
}

class USDollar extends Money {
  const CURRENCY = "$";

  public function toString() {
    $sign = ($this->value < 0 ? "-" : "");
    return $sign . self::CURRENCY . number_format(abs($this->value), 2);
  }
}

$pocket = new Pocket(0, "HUF");
echo $pocket->toString(), "\n";
echo $pocket->toString(USDollar::CURRENCY), "\n";

$pocket->addTransaction(new HungarianForint(1200));

echo $pocket->toString(), "\n";
echo $pocket->toString(USDollar::CURRENCY), "\n";

$pocket->addTransaction(new USDollar(-15.50));

echo $pocket->toString(), "\n";
echo $pocket->toString(USDollar::CURRENCY), "\n";

$pocket->addTransaction(new USDollar(17.20));

echo $pocket->toString(), "\n";
echo $pocket->toString(USDollar::CURRENCY), "\n";

echo "Transations:\n";
foreach($pocket->listTransactions() as $money) {
  echo "  ", $money->toString(), "\n";
}

0 HUF with 0 transaction
$0.00 with 0 transaction
 -D- Transaction added with value of (1 200 HUF).
1 200 HUF with 1 transaction
$5.04 with 1 transaction
 -D- Transaction added with value of (-$15.50).
-2 475 HUF with 2 transactions
-$10.40 with 2 transactions
 -D- Transaction added with value of ($17.20).
1 603 HUF with 3 transactions
$6.73 with 3 transactions
Transations:
  1 200 HUF
  -$15.50
  $17.20