Unless I'm missing something, I don't see the problem of having PHP return that information, since it already does so. E.g. __DIR__ will return:
/home/username/path/to/dir
This assumes your project code lives somewhere in a subfolder within `$HOME`. In many situations, that is not the case at all. For example, it could simply be `/app`. Or, as a result of a chroot, `/home/username` may not be exposed at all. Instead, `__DIR__` would simply return `/path/to/dir`.
Perhaps a magic constant would be the wrong way to do it, but in my thinking, it would fit right in with __FILE__, __DIR__, etc.
If it were to fit in with __FILE__, then it should be a magic constant. But I do not think it fits there. It should be an environment variable because it belongs to the runtime environment.