Derick Rethans has posted the first in a series focusing on the Xdebug tool and the code coverage functionality it can provide via PHPUnit's testing. In this first post he catches the reader up on the current state of things and what all the Xdebug tool can do.

Since ages Xdebug has provided code coverage support for PHPUnit, a way to show which lines are covered by your test cases. But I never really wrote about how it works. A recently filed bug prompted me to write this post, as well as a follow up post on Code Coverage's future.

He starts off with the early days of Xdebug, how it hooked into the Zend Engine (that powers a lot of PHP behind the scenes) and when it was triggered. This came with its own set of problems so Xdebug was updated to overload some opcodes. He talks about how it can calculate the unused lines and determines which lines can be covered in the code coverage results. He provides some example code showing the execution of the coverage report on a simple function and try/catch handler, complete with the HTML output of the results.