sample_test.php

<?php

class AddressBook
{
   private $users = [];
   
   public function addUser(UserInterface $user)
   {
      $user->registerAsAdded();
      
      if ($user->isLocal()) {
         $this->users[] = $user;
      }
   }
   
   public function getCount()
   {
      return count($this->users);
   }
}

class AddressBookTest extends TestCase
{
   private $addressBook;

   public function setUp()
   {
      $this->addressBook = new AddressBook();
   }

   public function testNonLocalUserIsNotAdded()
   {
      $userMock = $this->getMock('UserInterface');
      // Tell the mock to return FALSE for the "isLocal" method
      $userMock->expects($this->once())->method('isLocal')->will($this->returnValue(false));

      $this->addressBook->addUser($userMock);
      $this->assertEquals(0, $this->addressBook->getCount());  
   }
   
   public function testLocalUserIsAdded()
   {
      $userMock = $this->getMock('UserInterface');
      // Tell the mock to return TRUE for the "isLocal" method
      $userMock->expects($this->once())->method('isLocal')->will($this->returnValue(true));
      
      $this->addressBook->addUser($userMock);
      $this->assertEquals(1, $this->addressBook->getCount());
   }

   public function testUserGetsRegistered()
   {
      $userMock = $this->getMock('UserInterface');
      // Tell the mock that "registerAsAdded" will be called exactly once. When you 
      // use the "expects" method on a mock, you're actually setting up an explicit 
      // test expectation. Here we say "$this->once()" - but we could also pass in 
      // "$this->exactly(1)". If it's called *less* or *more* than once, this test will fail.
      $userMock->expects($this->once())->method('registerAsAdded');
      
      // Because we've set up expectations on our mock, we don't need assertions here
      $this->addressBook->addUser($userMock);
   }
}

Instead of instantiating the $userMock variable in our tests, we could have also done it in our setUp method and saved it to a class property.