gistfile1.php

<?php
class TestCase extends PHPUnit_Framework_TestCase
{
    private $dep;

    private $service;

    public function setUp()
    {
        $this->service = new Service();
        $this->dep = $this->getMock('Dependency');
    }

    public function testInvokeIsCalled()
    {
        $this->dep
            ->expects($this->once())
            ->method('invoke');

        $this->service->execute($this->dep);
    }
}

interface Dependency
{
    public function invoke();
}

class Service
{
    public function execute(Dependency $dep)
    {
        $dep->invoke();
    }
}