If you want to handle sessions with a class, I wrote this little class:

startSession();
        
        return self::$instance;
    }
    
    
    /**
    *    (Re)starts the session.
    *    
    *    @return    bool    TRUE if the session has been initialized, else FALSE.
    **/
    
    public function startSession()
    {
        if ( $this->sessionState == self::SESSION_NOT_STARTED )
        {
            $this->sessionState = session_start();
        }
        
        return $this->sessionState;
    }
    
    
    /**
    *    Stores datas in the session.
    *    Example: $instance->foo = 'bar';
    *    
    *    @param    name    Name of the datas.
    *    @param    value    Your datas.
    *    @return    void
    **/
    
    public function __set( $name , $value )
    {
        $_SESSION[$name] = $value;
    }
    
    
    /**
    *    Gets datas from the session.
    *    Example: echo $instance->foo;
    *    
    *    @param    name    Name of the datas to get.
    *    @return    mixed    Datas stored in session.
    **/
    
    public function __get( $name )
    {
        if ( isset($_SESSION[$name]))
        {
            return $_SESSION[$name];
        }
    }
    
    
    public function __isset( $name )
    {
        return isset($_SESSION[$name]);
    }
    
    
    public function __unset( $name )
    {
        unset( $_SESSION[$name] );
    }
    
    
    /**
    *    Destroys the current session.
    *    
    *    @return    bool    TRUE is session has been deleted, else FALSE.
    **/
    
    public function destroy()
    {
        if ( $this->sessionState == self::SESSION_STARTED )
        {
            $this->sessionState = !session_destroy();
            unset( $_SESSION );
            
            return !$this->sessionState;
        }
        
        return FALSE;
    }
}

/*
    Examples:
*/

// We get the instance
$data = Session::getInstance();

// Let's store datas in the session
$data->nickname = 'Someone';
$data->age = 18;

// Let's display datas
printf( '
My name is %s and I\'m %d years old.
' , $data->nickname , $data->age );

/*
    It will display:
    
    Array
    (
        [nickname] => Someone
        [age] => 18
    )
*/

printf( '
%s
' , print_r( $_SESSION , TRUE ));

// TRUE
var_dump( isset( $data->nickname ));

// We destroy the session
$data->destroy();

// FALSE
var_dump( isset( $data->nickname ));

?>

I prefer using this class instead of using directly the array $_SESSION.