Introduction to the MoneyPHP library and description of different issues caused by using FLOAT type to handle monetary amounts.

1. How to count money
and not lose it
Piotr Horzycki
www.peterdev.pl | twitter.com/peterdevpl

2. var_dump((int) ('4.20' * 100));
→ int(420)
var_dump((int) ('4.10' * 100));
→ int(409)
“I can set a product price to 4.20 PLN, but not 4.10
– it’s being changed to 4.09. Why?”
Bugs...

3. if (parseInt(amount) > 0) {
/* proceed with payment */
} else {
/* disable payment button */
}
Why does 0.5 not work?
Bugs...

5. <p>Price: <?php echo strtr($price, '.', ','); ?> PLN</p>
<p>Price: <?php echo str_replace($price, '.', ','); ?> PLN</p>
<p>Price: <?php echo number_format($price, 2, ',', ' '); ?> PLN</p>
Formatting price strings – the wrong way...

6. “A large proportion of the computers in this world
manipulate money, so it's always puzzled me that
money isn't actually a first class data type in any
mainstream programming language. The lack of a
type causes problems, the most obvious surrounding
currencies. (...) The more subtle problem is with
rounding. Monetary calculations are often rounded to
the smallest currency unit. When you do this it's easy to
lose pennies (or your local equivalent) because of
rounding errors.
The good thing about object-oriented programming is
that you can fix these problems by creating a Money
class that handles them. Of course, it's still surprising
that none of the mainstream base class libraries
actually do this.”
https://martinfowler.com/eaaCatalog/money.html

7. use MoneyCurrency;
use MoneyMoney;
// 5,00 USD
$fiver = new Money(500, new Currency('USD'));
// or shorter:
$fiver = Money::USD('500');
MoneyPHP: PHP implementation of the Money pattern

8. final class Money implements JsonSerializable
{
/** @var string */
private $amount;
/** @var Currency */
private $currency;
/** @var Calculator */
private static $calculator;
private static $calculators = [
BcMathCalculator::class,
GmpCalculator::class,
PhpCalculator::class,
];
/* … */
}

9. $value1 = Money::EUR(800);
$value2 = Money::EUR(500);
$value3 = Money::EUR(100);
$value1->add($value2);
$value1->subtract($value2, $value3);
$value1->multiply(2);
$value1->divide(2);
$value1->mod($value2); // 3.00 EUR
$value1->ratioOf($value2); // '1.6'
Basic money arithmetics

10. $value1 = Money::USD(800); // $8.00
$value2 = Money::USD(100); // $1.00
$result = $value1->isSameCurrency($value2); // true
$result = $value1->equals($value2); // false
$result = $value1->greaterThan($value2); // true
Comparing money objects

11. Immutability
$jimPrice = $hannahPrice = Money::EUR(2500);
$coupon = Money::EUR(500);
// wrong
$jimPrice->subtract($coupon);
$jimPrice->equals($hannahPrice); // true

12. Immutability
$jimPrice = $hannahPrice = Money::EUR(2500);
$coupon = Money::EUR(500);
// wrong
$jimPrice->subtract($coupon);
$jimPrice->equals($hannahPrice); // true
// correct
$jimPrice = $jimPrice->subtract($coupon);
$jimPrice->lessThan($hannahPrice); // true
$jimPrice->equals(Money::EUR(2000)); // true
$jimPrice and $hannahPrice are immutable value objects

13. use MoneyMoney;
$profit = Money::EUR(5); // 5 euro cents
list($my_cut, $investors_cut) = $profit->allocate([70, 30]);
// $my_cut is 4 cents, $investors_cut is 1 cent
Allocating profits

14. use MoneyMoney;
$profit = Money::EUR(5); // 5 euro cents
list($my_cut, $investors_cut) = $profit->allocate([70, 30]);
// $my_cut is 4 cents, $investors_cut is 1 cent
// The order is important:
list($investors_cut, $my_cut) = $profit->allocate([30, 70]);
// $my_cut is 3 cents, $investors_cut is 2 cents
Allocating profits

15. Database
INT, BIGINT…
VARCHAR...
DECIMAL (MySQL)
NUMERIC (Oracle)
FLOAT, DOUBLE...

16. Database
INT, BIGINT…
VARCHAR...
DECIMAL (MySQL)
NUMERIC (Oracle)
FLOAT, DOUBLE...
DECIMAL(10, 2)
total decimal
digits places
max 99,999,999.99

17. Exchange rates...
https://www.bankofengland.co.uk/boeapps/database/Rates.asp

18. https://en.wikipedia.org/wiki/Redenomination

19. https://www.theguardian.com/world/2018/aug/20/venezuela-bolivars-hyperinflation-banknotes

20. https://www.amusingplanet.com/2018/08/hungarys-hyperinflation-worst-case-of.html
Hungary’s hyperinflation in 1946

21. Converting currencies with MoneyPHP
use MoneyConverter;
use MoneyCurrency;
use MoneyExchangeFixedExchange;
$exchange = new FixedExchange([
'EUR' => [
'USD' => 1.25
]
]);
$converter = new Converter(new ISOCurrencies(), $exchange);
$eur100 = Money::EUR(100);
$usd125 = $converter->convert($eur100, new Currency('USD'));

22. Converting currencies with MoneyPHP and Swap
use MoneyMoney;
use MoneyConverter;
use MoneyExchangeSwapExchange;
use SwapBuilder;
$swap = (new Builder())
->add('fixer', ['access_key' => 'your-access-key'])
->build();
$exchange = new SwapExchange($swap);
$converter = new Converter(new ISOCurrencies(), $exchange);
$eur100 = Money::EUR(100);
$usd125 = $converter->convert($eur100, new Currency('USD'));

23. Save the conversion rate!

24. <p>Price: <?php echo strtr($price, '.', ','); ?> PLN</p>
<p>Price: <?php echo str_replace($price, '.', ','); ?> PLN</p>
<p>Price: <?php echo number_format($price, 2, ',', ' '); ?> PLN</p>
Price formatting again...

25. $100
€100
£100
¥100
...
Price formatting again...

26. $100
€100
£100
¥100
...
100 zł
Price formatting again...

27. $money = new Money(100, new Currency('USD'));
$currencies = new ISOCurrencies();
$numberFormatter = new NumberFormatter(
'en_US', NumberFormatter::CURRENCY
);
$moneyFormatter = new IntlMoneyFormatter(
$numberFormatter, $currencies
);
echo $moneyFormatter->format($money); // outputs $1.00

28. $money = new Money('12345678', new Currency('PLN'));
$currencies = new ISOCurrencies();
$numberFormatter = new NumberFormatter(
'pl_PL', NumberFormatter::CURRENCY
);
$moneyFormatter = new IntlMoneyFormatter(
$numberFormatter, $currencies
);
echo $moneyFormatter->format($money); // outputs 123 456,78 zł

29. https://en.wikipedia.org/wiki/Dollar_sign

30. $currencies = new ISOCurrencies();
$americanNumberFormatter = new NumberFormatter('en_US', NumberFormatter::CURRENCY);
$mexicanNumberFormatter = new NumberFormatter('es_MX', NumberFormatter::CURRENCY);
$dollarsFormatter = new IntlMoneyFormatter($americanNumberFormatter, $currencies);
$pesosFormatter = new IntlMoneyFormatter($mexicanNumberFormatter, $currencies);
$dollars = new Money(12345, new Currency('USD'));
$pesos = new Money(12345, new Currency('MXN'));
echo $dollarsFormatter->format($dollars) . PHP_EOL; // $123.45
echo $pesosFormatter->format($pesos); // $123.45
Be careful with parsing money strings!

31. Rounding
●
https://en.wikipedia.org/wiki/Cash_rounding
(aka Swedish rounding)
●
Polish income tax: round to the nearest whole zloty

32. Rounding
$money = new Money('12345', new Currency('USD'));
// $123.45 is the initial amount
$multiplied = $money->multiply('1.23', Money::ROUND_UP);
// $151.8435 before rounding
echo $dollarsFormatter->format($multiplied);
// output: $151.85

33. Architecture
final class Invoice
{
private $seller;
private $buyer;
private $issueDate;
private $dueDate;
/* … */
public addItem(Money $unitPrice, int $quantity, Tax $tax) {}
public getTotalAmount(): Money {}
public getTaxAmount(): Money {}
}

34. Architecture
interface InvoiceBuilder
{
fromSeller(Contractor $seller): self;
toBuyer(Contractor $buyer): self;
addItem(Money $unitPrice, int $quantity, Tax $tax): self;
/* … */
build(): Invoice;
}

35. Further reading
●
https://www.h-schmidt.net/FloatConverter/IEEE754.html
●
https://martinfowler.com/eaaCatalog/money.html
●
http://moneyphp.org/en/stable/
●
https://romaricdrigon.github.io/2019/07/05/value-objects-doctrine-and-symfony-forms
●
https://github.com/Sylius/SyliusMoneyBundle

36. Thank you!
Piotr Horzycki
www.peterdev.pl | twitter.com/peterdevpl