Sebastian Bergmann, the author of PHPUnit, recently had a blog post about Stubbing and Mocking Static Methods. With PHPUnit 3.5 and PHP 5.3 as well as consistent use of late static binding, you can do
$class::staticExpects($this->any())
->method('helper')
->will($this->returnValue('bar'));
Update: staticExpects
is deprecated as of PHPUnit 3.8 and will be removed completely with later versions.