Unit test cases in zend2 for service

I am writing unit test case for service in zend2. I have experience in zend2_ dcotrine2 But I am beginner in writing unit test cases.

Method in service for which I have to write unit test cases.

//function create repository

public function createrepo($clientId, $projectId, $username,$password)
{
if (0 === (int)$client) throw new InvalidArgumentException('CientId must be an integer value.');
if ('' === $username) throw new InvalidArgumentException('USername can not be empty.');
  // Decode client's/project Id
$clientId = base64_decode($clientId);
$projectId = base64_decode($projectId);

  // Make project's directory
$this->createProjectDir($clientId, $projectId);

  // Initialize Git within same project's directory
$this->initGitForProjectDir($clientId, $projectId);

  // Add user in Apache's known user list
$this->genHtPasswdFileForProjectDir($clientId, $username, $password);

  // Make HTTP Auth overridable from within project
$this->genHtAccessFileForProjectDir($clientId, $projectId);

return true;

}

My test cases are: a) Client id must be integer b) Username should not be empty. c) Function must return true.

I am not using any entity or entity repository in this project because it is not required in this application. I am using this function in Application/Service/Repository. Please give me an idea on code level.