Elastica\Query\QueryString::setDefaultOperator PHP Method

    public function setDefaultOperator($operator)
    {
        return $this->setParam('default_operator', $operator);
    }

 /**
  * @return ElasticaQuery
  */
 private function prepareQuery($params, $start = null, $limit = null)
 {
     $query = null;
     $filter = null;
     $sort = ['_score' => 'desc'];
     // We'd like to search in both title and description for keywords
     if (!empty($params['keywords'])) {
         $query = new QueryString($params['keywords']);
         $query->setDefaultOperator('AND')->setFields(['title', 'description']);
     }
     // Add location filter is location is selected from autosuggest
     if (!empty($params['location_id'])) {
         $location = Location::find($params['location_id']);
         $filter = new GeoDistance('location', ['lat' => $location->lat, 'lon' => $location->lon], $params['radius'] . 'mi');
         // Sort by nearest hit
         $sort = ['_geo_distance' => ['jobs.location' => [(double) $location->lon, (double) $location->lat], 'order' => 'asc', 'unit' => 'mi']];
     }
     // If neither keyword nor location supplied, then return all
     if (empty($params['keywords']) && empty($params['location_id'])) {
         $query = new MatchAll();
     }
     // We need a filtered query
     $elasticaQuery = new ElasticaQuery(new Filtered($query, $filter));
     $elasticaQuery->addSort($sort);
     // Offset and limits
     if (!is_null($start) && !is_null($limit)) {
         $elasticaQuery->setFrom($start)->setSize($limit);
     }
     // Set up the highlight
     $elasticaQuery->setHighlight(['order' => 'score', 'fields' => ['title' => ['fragment_size' => 100], 'description' => ['fragment_size' => 200]]]);
     return $elasticaQuery;
 }