Sebastian Bergmann has a new post to his blog talking about some of the future plans for better code coverage metrics for PHP applications (not just the statistics that we have now as generated from PHPUnit runs combined with Code_Coverage PEAR package and Xdebug).
Xdebug currently only supports what is usually referred to as Line Coverage. This software metric measures whether each executable line was executed. Based on the line coverage information provided by Xdebug, PHP_CodeCoverage also calculates the Function / Method Coverage software metric that measures whether each function or method has been invoked.
The various kinds of coverage they're planning the in future include statement coverage, branch coverage (boolean evaluation), call coverage, path coverage with an alternative of linear code sequence and jump coverage (LCSAJ).