gistfile1.php

class MyClass
{
  public function foo($paramA, $paramB)
  {
    return $paramA . $paramB;
  }
  
  public function bar($paramA, $paramB)
  {
      // more crap here i don't care about
      // .....
      
      // and then the important part
      return $this->foo($paramA, $paramB);
  }
}

class MyClassToMockTest
{
  public function getMyClassMock()
  {
    $myClass = new MyClass();
    
    $mockedClass = \Mockery::mock($myClass);
    
    $mockedClass->shouldRecevice('bar')
                ->with(\Mockery::any(), \Mockery::any())
                ->andReturnUsing( function($paramA, $paramB) {
                    // this will fail, so i am looking for a pointer on how to make it work
                    return $this->foo($paramA, $paramB);
                });
  }
}

That don't help much. I have no idea what's your real code. Also, if "bar" is "more crap here I don't care about" then why are you even testing it? Just test "foo" and that's it.