Of course, simple miss :). Thanks!
Another question that may not belong here. If it doesn't, then I can create a new entry.
Here is the code I am trying to get to work:
$UpdateQuery->bindColumn("BokningAvbokningAterbetalningsnummer", "i", "".((isset($_POST["AterbetalninSwishnr"]))?$_POST["AterbetalninSwishnr"]:"")||((isset($_POST["AterbetalninKontonr"]))?$_POST["AterbetalninKontonr"]:"") ."", "WA_DEFAULT");