maybe, but you need to understand the problem first:
first time the page is loaded, $_SESSION is empty.
on :36 - :49, you're trying to use $_SESSION["multi"] without first checking if it exists.
instead, you're checking $_POST.
dunno what's happening with the others (they seem to be in different files), but probably a similar situation.
i like to recommend writing out the logic of your app first - no details, no code - kindof like you're writing an outline for an essay or bullet points for instructions.
then keep breaking each step into individual instructions. make up names for those instructions. then, once all of this is done, write functions with those names, and write code in them which does those things.
then you're done.
(well, ok, lies. then you test, then you degub.)
then at some point you die