Pager::getOffsetByPageId() -- Returns offsets for given pageID.
Description
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
Return value
return array with first and last offsets