array(2) {
  [0]=>
  string(3) "bot"
  [1]=>
  string(5) "sysop"
}
array(2) {
  [0]=>
  string(3) "bot"
  [1]=>
  string(5) "sysop"
}
array(0) {
}

FROM
                $addedGroups = array();
                if ( $wasSysop ) {
                        $dbw->delete( 'user_groups',
                                array(
                                        'ug_user' => $id,
                                        'ug_group' => 'sysop',
                                ),
                                'DesysopForm::doSubmit' );
                        $addedgroups[] = "sysop";
                } elseif ( !$wasSysop ) {
                        $this->showFail( 'not_sysop' );
                        return;
                }

                $wgMemc->delete( "$dbName:user:id:$id" );

                //array_diff does the trick.
                $newGroups = array_diff($oldGroups, $addedGroups);

                var_dump($oldGroups);
                var_dump($newGroups);
                var_dump($addedGroups);