Index: includes/Setup.php
===================================================================
--- includes/Setup.php	(revision 15178)
+++ includes/Setup.php	(working copy)
@@ -129,7 +129,11 @@
 # If session.auto_start is there, we can't touch session name
 #
 if (!ini_get('session.auto_start')) {
-	session_name( $wgCookiePrefix . '_session' );
+	if ( $wgSessionName == '' ) {
+		session_name( $wgCookiePrefix . '_session' );
+	} else {
+		session_name ( $wgSessionName );
+	}
 }
 
 if( !$wgCommandLineMode && ( isset( $_COOKIE[session_name()] ) || isset( $_COOKIE[$wgCookiePrefix.'Token'] ) ) ) {
Index: includes/DefaultSettings.php
===================================================================
--- includes/DefaultSettings.php	(revision 15178)
+++ includes/DefaultSettings.php	(working copy)
@@ -1101,6 +1101,7 @@
  * Set to set an explicit domain on the login cookies eg, "justthis.domain. org"
  * or ".any.subdomain.net"
  */
+$wgSessionName = ''; 
 $wgCookieDomain = '';
 $wgCookiePath = '/';
 $wgCookieSecure = ($wgProto == 'https');