In ZE / PHP 4 objects weren't references, so when they were passed to a function a copy was made. amount = $cash; } } function deposit($account, $cash) { $account->amount += $cash; } function withdraw(&$account, $cash) { $account->amount -= $withdraw; } $wallet = new bank_account(100); deposit($wallet, 50); withdraw($wallet, 120); echo "The account balance is: {$this->amount}"; ?>]]> The account balance is: -20 Since %deposit()% wasn't pass-by-reference, the extra deposit was made to a _copy_ of the bank account, which was then destroied when the method returned.