PHPUnit overwrite actual method with stub

I just started playing around with PHPUnit and I was wondering if it is possible to overwrite/replace a method with a stub. I have some experience with Sinon, and with Sinon this is possible (http://ift.tt/1kUFzRX)

I want to something like this:

<?php

class Foo {

  public $bar;

  function __construct() {
    $this->bar = new Bar();
  }

  public function getBarString() {
    return $this->bar->getString();
  }

}

class Bar {

  public function getString() {
    return 'Some string';
  }

}



class FooTest extends PHPUnit_Framework_TestCase {

  public function testStringThing() {
    $foo = new Foo();

    $mock = $this->getMockBuilder( 'Bar' )
      ->setMethods(array( 'getString' ))
      ->getMock();

    $mock->method('getString')
      ->willReturn('Some other string');

    $this->assertEquals( 'Some other string', $foo->getBarString() );
  }

}

?>