undefined variable

I'm doing something like this: $PTR_fcc = isset($_POST['PTR_fcc'])==true ? $_POST['PTR_fcc'] : 0; and possibly (maybe not, depends on some logic): $PTR_fcc = ($fcco==0 || $mboxid==$inboxid || $mboxid==$outboxid || $mboxid==$junkboxid || $mboxid==$trashboxid) ? 0 : $mboxid; and then later I do: $str = "fcc: " . $PTR_fcc; Now, upon examination it appears that I have an instance where $str ends up as: fcc: undefined My question is, under what conditions can a variable be set by the PHP system to the string "undefined"? Would this require that my ajax call in the browser would have to have had something explicit like: ... &PTR_fcc=undefined ... -- Cheers -- Tim