Line 21 is:
$WAFV_Errors .= WAValidateRQ(((isset($_POST["event"]))?$_POST["event"]:"") . "",false,5);
You are trying to treat event like a string instead of array on that line. I think you should use:
$WAFV_Errors .= WAValidateRQ(((isset($_POST["event"]))?implode(",",$_POST["event"]):"") . "",false,5);