Very interesting presentation of Juan Treminio about unit tests and PHPUnit: PDF. He explain how to generate mocks and stubs using the MockBuilder:

• Generate a basic mock, without call setMethods() method:
  * All methods in mocked object are stubs
  * Return null
  * Methods easily overridable

  $payment = $this->getMockBuilder('AuthorizeNetAIM') ->getMock();

• Pass an empty array
  * Same as if not calling setMethods()
  * All methods in mocked object are stubs
  * Return null
  * Methods easily overridable

  $payment = $this->getMockBuilder('AuthorizeNetAIM') ->setMethods(array()) ->getMock();

• Pass null
  * All methods in mocked object are mocks
  * Run actual code in method
  * Not overridable

  $payment = $this->getMockBuilder('AuthorizeNetAIM') ->setMethods(null) ->getMock();

• Pass an array with method names
  –Methods identified are stubs
  • Return null
  • Easily overridable
  –Methods *not* identified are mocks
  • Actual code is ran
  • Unable to override

  $payment = $this->getMockBuilder('Payment') ->setMethods( array('authorizeAndCapture',) ) ->getMock();

Source – Juan Treminio presentation
Source – Juan Treminio blog