link problem

I have lots of folders and files some are in the main public_html folder and some are in their own foder. I need to find a way to make all the links relative to the public_html folder For example if I am in the 'gallery' folder, the main index file is located at ../index, however if I om in the contact page which is in the same directory as index the link is just index.php. I want to have one menu and make all the links work without having to enter a full http://www.mydomain/myfolder/myfile.php. I tried $_SERVER['doc_root']; but this gives the full server root. I am sure I have see a way to define directorries using define() or something. Ta.