You introduced a new variable $query which doesn't exist. Changed it to $string as in the rest of the code.
Here it happened: https://github.com/dmj/vf2/commit/e6423f1f96c45411d4b42fce51ff41c20894aa22

Thanks to David Maus.

* VuFind/Search/BackendManager.php (attachShared, detachShared):
  New functions. Implement SharedListenerAggregateInterface.
  ($listeners): New property. Attached listeners.
  (__construct): Initialize listeners property.

* VuFindSearch/Backend/Solr/Backend.php (setQueryBuilder): Typehint on
  QueryBuilderInterface.
* VuFindSearch/Backend/Solr/QueryBuilder.php: Implements
  QueryBuilderInterface, remove old @todo markers.
* VuFindSearch/Backend/Solr/QueryBuilderInterface.php: Define
  interface for SOLR query builders.

Resolves VUFIND-663.

Thanks to Benjamin Mosior.

Resolves VUFIND-744; thanks to Tom Misilo.

Some edge cases won't be fixed until next SolrMarc upgrade (need List support)

* VuFindSearch/Backend/Solr/HandlerMap.php (setHandlerMap): Set D/I/A
  if handler definition does not contain functions.

Extra safety check for RSS detection.

* VuFindSearch/Backend/Solr/Backend.php (search): Use dedicated
  variable for E_STRICT compliance.

* VuFind/Search/Factory/AbstractSolrBackendFactory.php
  (createListeners): Create and attach SOLR v4 error listener.