To pass the tid as an array value in callback funtion
I am sharing here some dummy data with you, it might help you.



{
    "tid": "25,17,28",
    "pager": 1,
    "limit": 5,
    "order": "desc"
}

 

   array(
                  'name'         =&gt; 'order',
                  'type'         =&gt; 'string', // it should be string because it can be either ASC or DESC
                  'description'  =&gt; t('The order clause for the query that retrieve the nodes.'),
                  'source'       =&gt; array('data' =&gt; 'order'),
                  'optional'     =&gt; TRUE,
                  'default value'=&gt; array('t.sticky' =&gt; 'DESC', 't.created' =&gt; 'DESC'),
                ),


// Call back function 

function taxonomy_service_select_nodes($tid, $pager, $limit, $order) {
  $tid_collection = array();
  $tid_collection = explode(',', $tid); // Here your tids are in array format and you can use in your code accordingly.
  echo $pager;
  echo $limit;
  echo $order;
}