BankAccount1.php

<?php
class BankAccount
{
  private $twitter;
  public function __construct(Twitter $twitter) 
  {
    $this->twitter = $twitter;
  }
  public function deposit($amount){
  }
}

BankAccount2.php

<?php
class BankAccount
{
  // ...
  public function deposit($amount)
  {
    $this->twitter->tweet("Yay, someone deposited $amount");
  }
}

BankAccountTest1.php

<?php
class BankAccountTest extends PHPUnit_Framework_TestCase
{
  public function testSendEmailWhenReceivingMoney()
  {
    $twitter = $this->getMock('Twitter');
    $account = new BankAccount($twitter);
    $account->deposit(10);
  }
}

BankAccountTest2.php

<?php
class BankAccountTest extends PHPUnit_Framework_TestCase
{
  public function testSendEmailWhenReceivingMoney()
  {
    $twitter = $this->getMock('Twitter');
    $twitter->expects($this->once())
      ->method('tweet');
    $account = new BankAccount($twitter);
    $account->deposit(10);
  }
}

Twitter.php

<?php
interface Twitter
{
  function tweet($message);
}