why is the logic reversed?

that is a good question

This still seems questionable to me:

$is_absolute = self::isRelativePath($path);

Specifically, this code is good if you rename the method to isAbsolutePath(), or this method name is fine if you invert the logic and invert the return value before assigning. But this is setting $is_absolute = is_relative(), which doesn't make sense, and isRelativePath() is returning true for absolute (i.e., not relative) paths.