Hi Sunil
AOLserver/3.5.11 is not *that* old that it should have a different way of writing headers.
One thing you could try is to search your existing codebase for examples of how headers are set. Presumably it's working, so it is just a matter of finding it. For example, search for where cookies are created. If you are not using OpenACS, the headers might be written with just a raw ns_write.
Let us know if you need more help
Brian