Psalm - a static analysis tool for PHP

<?php
  
/** @psalm-immutable */
abstract class MoneyAmount {
  public float $value;
  
  public function __construct(float $value) {
    $this->value = $value;
  }
  
  /** @psalm-pure */
  abstract public static function getSymbol() : string;
  
  /**
   * @param static $amount
   * @return static
   */
  public function add(self $amount) : self {
    return new static($amount->value + $this->value);
  }
  
  public function getFormatted() : string {
   	return static::getSymbol() . number_format($this->value, 2); 
  }
}

class GbpAmount extends MoneyAmount {
  public static function getSymbol() : string {
    return '£';
  }
}

/** @psalm-immutable */
class UsdAmount extends MoneyAmount {
  public static function getSymbol() : string {
    return '$';
  }
  
  public function convertToGbp(float $exchange_rate) : GbpAmount {
    return new GbpAmount($this->value * $exchange_rate);
  }
}

/** @psalm-pure */
function addSalesTaxAndConvertToGbp(
  UsdAmount $usd,
  float $exchange_rate
) : GbpAmount {
  return $usd
    ->add(new UsdAmount($usd->value * 0.08))
    ->convertToGbp($exchange_rate);
}

$usd_amount = new UsdAmount(5.0);
echo addSalesTaxAndConvertToGbp($usd_amount, 1.20)->getFormatted();

Snippet created on September 10 2019 at 04:35 UTC