Solr Extension - Changing the wt-Parameter from xml to json

Hi I'm using the Solr extension (http://php.net/manual/en/book.solr.php) to create a search site for a company. I am trying figure out a way of changing the "wt"-parameter from "xml" to "json". I have searched already but with no success so far. Further information to help with my question: - http://wiki.apache.org/solr/CoreQueryParameters#wt - Example 5 on http://www.php.net/manual/en/solr.examples.php shows the "wt"-parameter set to "xml". Thanks in advance! tj