Base PHPRtfLite only supports landscape for the entire document. Attached patch adds _isLandscape to sections. And prints \lndscpsxn when rendering the section with the option set. Solution might not be the most elegant since the document wide option could conflict, I haven't tested this as we don't use it.
Tested by generating a document with a portrait page, then a landscape page, and then another portrait page. Generated document renders (and more importantly prints) correctly from office 2007 and OpenOffice 3.0