code: public function set($key, $value) {
          $this->open();
  
          $this->_data[$key] = $value;
  
          return $this;
      }
docs: Set a variable in the session array.
  
  $session->set('foo', 'bar');
  
  @param   string $key variable name
  @param   mixed $value value
  @return  $this