You can't, not give out directory information in the source code.
If you think directory locations is what hackers spend their time looking for you're misinformed...
There's plenty of other things to worry about, such as 777 permissions on directories that shouldn't be writable and so on..
Lots of ways to determine where files reside. Easiest way is to make a direct request to a file that is normally part of an include, there's usually at the very least 1 file floating around that doesn't check to make sure it wasn't requested directly.. As soon as that file is requested a visitor sees an error, along with your directory structure.
What is it you think a hacker can do by knowing simply where files reside?