> I found out the hard way that a sorted node-set is not iterated in  
> the sorted order,

XSLT has sets, "sets" as always denotes a datatype with no order
property, the set {1,2,3] is the same as the set {2,3,1}.

XSLT2 changes the underlying datatype to sequences which do have an
order, which is sometimes useful when processing sorted data.

> (the libxslt implementation used in PHP  
I believe libxslt supports teh node-set extension function which means
that you can create a document with things in the right order and then
process that (in-memory) document without having to serialise and
re-parse it. This allows you to do a two-stage process, forst sort and
filter then process, without having to use two explict transformation
steps.

David