I've looked into the source code and it's possible however not very cleary:
$client = new SolrClient($options);
$client->setServlet(SolrClient::SEARCH_SERVLET_TYPE, "request_handler_name");
Perhaps it's worth adding it on example page (
http://www.php.net/manual/en/solr.examples.php).
Speaking about the example page I have one suggestion. The first example which shows how to insert a document would benefit from adding one more line
$client->commit().
WIthout commit the document will obviously not get into Solr which might confused less experienced users.