PHPUnit test suite include path

Using phpunit and I am having some trouble with include paths, not for phpunit itself, but for my code and tests directory.

I have the following code structure:

Application
  -StringCalculator.php

tests
  -StringCalculatorTest.php

Inside my StringCalculatorTest.php i have a require statement:

require_once('../StringCalculator.php');

Running phpunit StringCalculatorTest.php from inside the tests folder works perfectly.

However, when i then introduce a phpunit.xml configuration file in the root directory i.e.

Application
  -StringCalculator.php

tests
  -StringCalculatorTest.php

phpunit.xml

the include path is screwed. I have to replace the require_once to

require_once('StringCalculator.php');

What is the correct way to set include paths between the application and the test directory?