Here is a good description how to use XDebug and PHPUnit to measure coverage of your codebase with unit-tests. In my case it gave me motivation to write more and even better tests in order to reach somewhere near 100% coverage.
Visit Matthew Weier O’Phinney’s Blogpost and complete your test suites.
My first shot looked like this:
phpunit --report _coverage/ FrameworkAllTests.php