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Sorry I'm not using PHPUnitbut Simpletest which is more simple to start with as It comes with an builtin tool to run tests from a browser quite easily.
I have made a repository to keep traces of my unit tests.
See next link :
http://xoops-unit-test.googlecode.com/svn/trunk/test/It is simple to translate from simpletest to PHPUnit the test scripts. The most difficult seems form me, to install PHPUnit framework on my computer and to use it.
Just install the files in the Xoops Root with docs, extra and htdocs. And point the browser to test directory.
Nota: One have to install simpletest framework directly from their web site. The version is 1.1.0 in 2010 but is stable and does the expected job. Install in the root xoops directory.