(PECL solr >= 0.9.2)
SolrQuery::setTermsSort — Specifies how to sort the returned terms
If SolrQuery::TERMS_SORT_COUNT, sorts the terms by the term frequency (highest count first). If SolrQuery::TERMS_SORT_INDEX, returns the terms in index order
sortTypeSolrQuery::TERMS_SORT_INDEX or SolrQuery::TERMS_SORT_COUNT
Returns the current SolrQuery object, if the return value is used.