<?php 

// meh
$mock
  ->method('sum')
  ->withConsecutive([1,2,3], [4,5,6])
  ->willReturnOnConsecutiveCalls(6, 15);

// instead?
$mock->method('sum')->with(1,2,3)->willReturn(6);
$mock->method('sum')->with(4,5,6)->willReturn(15);