<?php
$ltActive = null;
$rtActive = null;
if($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;