<?php class BankAccount { protected $balance = 0; public function getBalance() { return $this->balance; } protected function setBalance($balance) { if ($balance >= 0) { $this->balance = $balance; } else { throw new RuntimeException; } } public function depositMoney($balance) { $this->setBalance($this->getBalance() + $balance); return $this->getBalance(); } public function withdrawMoney($balance) { $this->setBalance($this->getBalance() - $balance); return $this->getBalance(); } }