<?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 ( ) ; } }