The Currency and Money Value Objects are probably the most used examples for explaining Value Objects, thanks to the Money pattern. This design pattern provides a solution for modeling a problem that avoids a floating-point rounding issue, which in turn allows for deterministic calculations to be performed.

In the real world, a currency describes monetary units in the same way that meters and yards describe distance units. Each currency is represented with a three-letter uppercase ISO code:

class Currency 
{ 
    private $isoCode;

    public function __construct($anIsoCode)
    {
        $this->setIsoCode($anIsoCode);
    }

    private function setIsoCode($anIsoCode)
    {
        if (!preg_match('/^[A-Z]{3}$/', $anIsoCode)) {
           throw new InvalidArgumentException();
        }

        $this->isoCode = $anIsoCode;
    }

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

One of the main goals of Value Objects is also the...