Consecutive calls mocking /w phpspec

FooSpec.php

<?php

/**
 * What I want to achieve
 */
class FooSpec
{
    function it_does_stuff($collab)
    {
        $collab->doCollabStuf()->willReturn('foo'); // Returns foo the first time
        $collab->doCollabStuf()->willReturn('bar'); // Returns bar the second time
                                                    // Returns null the third time
        
        $this->doStuff()
    }
}

/**
 * What I usually do
 */
 class FooSpec
{
    function it_does_stuff($collab)
    {
        $returns = ['foo', 'bar']
        $collab->doCollabStuf()->will(function() use (&$returns) {
            return array_shift($returns);
        });
        
        $this->doStuff()
    }
}

Thanks for the explanation @everzet.

However, I'm not sure to fully understand what you mean by "command", I've found an article of Martin Fowler about CQRS (http://martinfowler.com/bliki/CQRS.html).
I'll read it when I have enough time.

commands are things that permit to change state, while queries are here to read (eventually) current state.