great for adding unintended parameters in other functions like facet.contains 

?php 
$facet = 'title'  ; 
$str='isto'; 
$query = new SolrQuery($filter);
$query->setFacet(true)->setShowDebugInfo(true);

$query->addFacetField($facet)->setFacetMinCount(1)->setFacetLimit(200)->setfacetsort(SolrQuery::TERMS_SORT_INDEX);

$query->addparam('facet.contains',$str) ;

 ?>
// result of $query->getparams()  //check the lastone 
array(8) {
  ["q"]=>  array(1) {
    [0]=>    string(20) "collection_id_str:61"
  }
  ["facet"]=>  array(1) {
    [0]=>    string(4) "true"
  }
  ["debugQuery"]=>  array(1) {
    [0]=>    string(4) "true"
  }
  ["facet.field"]=>  array(1) {
    [0]=>    string(11) "440a_str_mv"
  }
  ["facet.mincount"]=>  array(1) {
    [0]=>    string(1) "1"
  }
  ["facet.limit"]=>  array(1) {
    [0]=>    string(3) "200"
  }
  ["facet.sort"]=>  array(1) {
    [0]=>    string(5) "index"
  }
  ["facet.contains"]=>  array(1) {
    [0]=>    string(4) "isto"
  }
}