Well, actually it's easier to call the same page every time, and use the variables to define the contents of the page, such as...
index.php?mode=add
index.php?mode=edit
index.php?mode=delete
Then on your index page, you could use something like:
<?
if($mode == "add"):
// this would be the add contents, or include, or whatever
elseif($mode == "edit"):
// this would be the edit contents, or include, or whatever
elseif($mode == "delete"):
// this would be the delete contents, or include, or whatever
else:
// this would be the default contents, or include, or whatever
endif;
?>
Or you could use case/switch. This basically creates a template for you, and all you have to worry about is the actual content, once the rest is in place.
You could use multiple pages called the way Shrike has here as well. Really it's up to you. If you already have all of those pages built, then that may be the best route, but if the pages aren't built yet, then you may want to look into the alternatives mentioned.