array Pager::getOffsetByPageId (
Returns offsets for given pageID . Eg, if you pass it pageID one and your perPage limit is 10 it will return (1, 10). pageID =2 would give you (11, 20).
if the method is called without parameter, pageID is set to currentPage
integer $pageid - PageID to get offsets for
return array with first and last offsets