accounts.php

<?php

class CheckingAccount extends BankAccount
{
}

class SavingAccount extends BankAccount
{
}

BankAccount-1.php

<?php

class BankAccount
{
    public function debit($amount)
    {
        //...
    }
    public function credit($amount)
    {
        //...
    }
}

BankAccount.php

<?php

abstract class BankAccount
{
    abstract public function debit($amount);
    abstract public function credit($amount);
}

BankAccountInterface.php

<?php

interface BankAccountInterface
{
    function credit($amount);
    function debit($amount);
}

class BankAccount implements BankAccountInterface
{
    //...
}

PaymentGateway.php

<?php

class PaymentGateway
{
    public function charge($amount, BankAccount $account)
    {
        // make a call to http web service here...
        $account->credit($amount)
    }
}

SomeAccount.php

<?php

class SomeAccount extends BankAccount
{
    public function credit($amount, $someOtherParameter)
    {
        //...
    }
}