jquery & php + if session is dead an error is returned

jquery & php + if session is dead an error is returned

I've noticed if the session is dead (on an Jquery AJAX PHP Request) the
data returned is preceeded with an error message if the session is needed
in the request.
How do other sites deal with this?
Similar code is shown below - eg: its using a SESSION variable in the code
which doesn't exist as the session is dead.
public function internal($variable) {
if($_SESSION['data'] == $variable) {
echo TRUE;
}else{
echo FALSE;
}
}
Should I use isset to check if the variable exists?
Should I be coding for dead sessions?
thx