<?php
$ltActive = 1;
$rtActive = 1;
if($ltActive && $rtActive){
$class = 'class when both sidebars are active';
}elseif(!$ltActive && !$rtActive){
$class = 'class when both sidebars are inactive';
}else{
$class = 'class when only one of the sidebars is active (either one)'; // <==
}
print $class;