spec

<?php

namespace spec\SO\Entity;

use
    PHPSpec2\ObjectBehavior,
    SO\Entity\Money as M;

class Money extends ObjectBehavior {

    function it_should_be_initializable()
    {
        $this->shouldHaveType('SO\Entity\Money');
    }

    function let()
    {
        $this->beConstructedWith('99.9900');
    }

    function it_represents_a_monetary_amount()
    {
        $this()->shouldBe('99.9900');
    }

    function it_adds()
    {
        $result = $this->add(new M('11.11'));

        $result()->shouldBe('111.1000');
    }

    function it_subtracts()
    {
        $result = $this->subtract(new M('11.11'));

        $result()->shouldBe('88.8800');
    }

    function it_multiplies()
    {
        $result = $this->multiply(1.16);

        $result()->shouldBe('115.9884');
    }

    function it_divides()
    {
        $result = $this->divide(1.16);

        $result()->shouldBe('86.1983');
    }

    function it_formats()
    {
        // 86.1983
        $this->divide(1.16)->format()->shouldBe('86.20');
    }

    function it_pretty_formats()
    {
        // 86.1983
        $this->divide(1.16)->format(TRUE)->shouldBe('86.20');
    }

    function it_cannot_pretty_format()
    {
        // 100.0000
        $this->add(new M(0.01))->format(TRUE)->shouldBe('100');
    }

    function it_formats_thousands()
    {
        $this->multiply(11111)->format()->shouldBe('1,110,988.89');
    }

}

src

<?php

namespace SO\Entity;

class Money {

    const PRECISION = 4;

    protected
        $integer_representation,
        $decimal_separator,
        $thousands_separator;

    public function __construct($amount, $decimal_separator = '.', $thousands_separator = ',')
    {
        $this->integer_representation = ((double) $amount) * $this->representation_factor();
        $this->decimal_separator = $decimal_separator;
        $this->thousands_separator = $thousands_separator;
    }

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

    public function __to_string()
    {
        return number_format($this->integer_representation / $this->representation_factor(),
            self::PRECISION, $this->decimal_separator, '');
    }

    public function add(Money $other)
    {
        $this->integer_representation += $other->integer_representation;

        return $this;
    }

    public function subtract(Money $other)
    {
        $this->integer_representation -= $other->integer_representation;

        return $this;
    }

    public function multiply($factor)
    {
        $this->integer_representation *= $factor;

        return $this;
    }

    public function divide($divisor)
    {
        $this->integer_representation /= $divisor;

        return $this;
    }

    public function format($pretty = FALSE)
    {
        $formatted = number_format($this(), 2, $this->decimal_separator, $this->thousands_separator);

        if ($pretty && preg_match('/\.00$/', $formatted))
        {
            $formatted = number_format($this(), 0, $this->decimal_separator, $this->thousands_separator);
        }

        return $formatted;
    }

    private function representation_factor()
    {
        return 10 ^ Money::PRECISION;
    }

}