array Pager_Sliding::getOffsetByPageId (
Eg, if you pass it
pageID = 5 and your
delta is 2 it will return you 3 and 7.
PageID of 6 would give you 4 and 8
NB: The behaviour of this function could be misleading: it was left only
for compatibility with PEAR::Pager.
It could raise some confusion when pageID is within
positions from an extreme: in fact this method returns also the extremes, while
$this->_getPageLinks leaves them out.
Pager_Sliding works this way: if pageID is NOT an extreme, show first and
last page within brackets:  << 5 | _6_ | 7 >> 
So when dealing with pageID within delta positions from an extreme,
this method would return the extreme as well, while $this->_getPageLinks
would return (for instance) 2 | _3_ | 4 | 5 even if pageID is 3 and delta
is 2. Consider this method deprecated and/or subject
integer $pageid - PageID to get offsets for
return array with first and last offsets