May need to run commit() after a delete query.
$result = $this->solrClient->commit();
(PECL solr >= 0.9.2)
SolrClient::deleteById — Delete by Id
Deletes the document with the specified ID. Where ID is the value of the uniqueKey field declared in the schema
The value of the uniqueKey field declared in the schema
Returns a SolrUpdateResponse on success and throws a SolrClientException on failure.