The Value Object Pattern By PHP

class BadDollar {
    protected $amount;
    public function __construct($amount=0) {
        $this->amount = (float)$amount;
    }
    public function getAmount() {
        return $this->amount;
    }
    public function add($dollar) {
        $this->amount += $dollar->getAmount();
    }
}

class Work {
    protected $salary;
    public function __construct() {
        $this->salary = new BadDollar(200);
    }
    public function payDay() {
        return $this->salary;
    }
}

class Person {
    public $wallet;
}

function testBadDollarWorking() {
    $job = new Work;
    $p1 = new Person;
    $p2 = new Person;
    $p1->wallet = $job->payDay();
    $this->assertEqual(200, $p1->wallet->getAmount());
    $p2->wallet = $job->payDay();
    $this->assertEqual(200, $p2->wallet->getAmount());
    $p1->wallet->add($job->payDay());
    $this->assertEqual(400, $p1->wallet->getAmount());
    //this is bad — actually 400
    $this->assertEqual(200, $p2->wallet->getAmount());
    //this is really bad — actually 400
    $this->assertEqual(200, $job->payDay()->getAmount());
}