Pathes (again...)

Hello ! I got a PHP programm which consist of multiple files and classes, which include each other. I have no access to the php-config-files. I wrote all includ-statements so, that they are relative to the domain-root: include $DOCUMENT_ROOT."/inc/some.php" for example. So all include-pathes are given relative to www.mydomain.com/ Now I want to port the programm, that it can be run in ANY PATH of the domain (like www.mydomain.com/path/ or www.mydomain.com/path/subpath/anothersubpath/), because I have customers, who only owns a path in the domain, not the domain. That means, too, that they have no access to the files in www.mydomain.com/. Whats the best way to do it WITHOUT changing every include-statement everytime a customer wants to use the programm in another subpath ? Greetings Martin