i suppose the scripts have worked any time and you are having problems moving to another site or something like that... (just to discard rewritting code)
Edit add.php and look for any lines like this
include ("xxxxx.php"); // or whatever
check if that files mentioned exist in the path they are supposed to exist. I mean: if the "include" lines are this way:
include ("/home/html/functions.php");
check if the file functions.php is located in /home/html/
Check if there is any config file ("config.php","xxxx.conf") and set proper values to the variables i.e new paths... i can't tell you more without anything else.