Use for both renaming and moving files, rename() takes two parameters: the original filename and the new filename you wish to use. Rename() can rename/move files across directories and drives, and will return true on success or false otherwise.
If you had $filename set to c:\\windows\\myfile.txt, the above script would move that file to c:\\windows\\myfile.txt.old.
Author's Note: rename() should be used to move ordinary files, and not files uploaded through a form. The reason for this is because there is a special function, called move_uploaded_file(), which checks to make sure the file has indeed been uploaded before moving it - this stops people trying to hack your server into making private files visible. You can perform this check yourself if you like by calling the is_uploaded_file() function.