The situation of having both a/b_foo.php and a/b/bar.php happens during refactoring when some of the classes have been moved under a/b and some not.

The current behavior is not user friendly - php says class is not found where it was found just fine in the previous version of the tree and the class has not changed.

Assuming we own phpbb_ namespace it may be ok to always try searching for a/b_foo.php. Otherwise maybe define a constant in test suite, if it is defined do the second loading pass and print a warning if any files are found there.