This is a general question. What are some techniques you employ to prevent the links in your PHP code from breaking every time you move a file or move a file into a different directory?
For front end stuff, always use absolute URLs (start with '/' so you can move from domain to domain as needed).
For internal include() / require() type stuff, do as Gaurav suggests and use a config file that creates a constant to represent your path (so you can change it easily as needed from one location in your code).
For library type stuff (i.e. classes, functions, etc) that you want to reuse, I would add that to the include_path either via php.ini (global or local), .htaccess (if you are using apache) or via the ini_set() function. That way you can include these files by just the filename (i.e. <?php include_once('library.php'); ?>)
If you go the ini_set route, take a look at the auto_append directive (which in turn can be set via php.ini, .htaccess or ini_set)... that way you can have a 'bootstrap' file added to every page request that sets up your include_path for just that application without having to add the ini_set statement every where you turn.
With all that said, I recommend that you:
Good luck!