PHP has a bunch of [[http://php.net/manual/en/language.constants.predefined.php | magic constants]] (such as `__FILE__` and `__DIR__`). The use of these constants can possibly cause issues if the magic constant does not exist in the targeted PHP version. For example, `__DIR__` wasn't introduced until PHP 5.3.0 but there is no linter step which checks whether `__DIR__` is used in rP, rARC or rPHU (all of which target PHP 5.2.3).
It should be reasonably straight-forward to add these magic constants to `resources/php_compat_info.json` (which lives in rPHU) so that we can detect this stuff during linting.