PHPUnit test doubles

I am starting to use PHPUnit for test my code but I have some problems with understand double tests.

I try to stub a class method b to return true instead of usual behavior (false) when is called since another method

I have a code like this

class MyClass {
    function a()
    {
        return $this->b();
    }

    function b() 
    {
        return false;
    }
}

class MyClassTest extends TestCase
{
     function testAThrowStubB()
     {
        $myClassStub = $this->getMockBuilder('\MyClass')
                              ->getMock();

        $myClassStub->expects($this->any())
                    ->method('b')
                    ->willReturn(true);

        // this assert will work
        $this->assertTrue($myClassStub->b());
        // this assert will fail
        $this->assertTrue($myClassStub->a());
     }
}

I thought my second assertion will work but it doesn't. I'm wrong and it is not possible? There is another way to test a function who depends on another overriding his behavior?

Thanks