includes linking to other files, help with relatives

I am creating a database connection path that check for a session variable to get the username, password, dbname, etc., but if the session has not been set yet, I need my script to get those values from another file (KEY.txt) where they are saved. I want to access this class from two directories, and this text file (KEY.txt) is in a directory above both. Like this: root: folder1 - folder2 folder1: KEY.txt folder2: index.php - database.php - folder3 folder3: index.php I need to use database.php in both index files, but database.php references KEY.txt. So my question is, do I have to check which directory the calling script is in (ie. which index.php file is using database.php) and add the according up directory references, or do I need to use some kind of $_SERVER[DOCUMENT_ROOT] thing? Any help would be appreciated!