PHPUnit - creating Mock objects to act as stubs for properties

I'm trying to configure a Mock object in PHPunit to return values for different properties (that are accessed using the __get function)

Example:

class OriginalObject {
 public function __get($name){
switch($name)
 case "ParameterA":
  return "ValueA";
 case "ParameterB":
  return "ValueB";
 }
}

I'm trying to mock this using:

$mockObject = $this->getMock("OrigionalObject");

$mockObject    ->expects($this->once())
    ->method('__get')
    ->with($this->equalTo('ParameterA'))
    ->will($this->returnValue("ValueA"));

$mockObject    ->expects($this->once())
    ->method('__get')
    ->with($this->equalTo('ParameterB'))
    ->will($this->returnValue("ValueB"));

but this fails horribly :-(