TestHelper should be require_once'd in any test suite that may be run independently.

Otherwise, Zend library will not be available in current include_path and you'll not be able to run a class specific unit test.

Let me show you a few examples:

// the following statement works fine phpunit --verbose Zend_Service_Yahoo_AllTests

// the following statements fail // Failed opening required 'Zend/*' ... phpunit --verbose Zend_Service_Yahoo_OfflineTest phpunit --verbose Zend_OpenId_ConsumerTest

// the following statement fails for the same reason as above phpunit --verbose Zend_OpenId_AllTests