In the config file, add mode to the PHPDIG_SESSID_VAR constant.
Code:
define('PHPDIG_SESSID_REMOVE',true); // remove SIDS or vars from indexed URLS
define('PHPDIG_SESSID_VAR','PHPSESSID,s'); // name of SID or variable to remove
// can be 's' or comma delimited 's,id,var,foo,etc'