discerning article threading?

[Paul Reilly]

Hi,

Does anyone know how I can discern how articles are threaded?

Popular newsreaders, show you the original message, and any replies in a
threaded manner. Sometimes the reply can be to a reply, and not to the
original message. So you see a structure like the following...

  o  my original post
  |
  |____reply A
  |
  |____reply B
  |    |___reply to reply B
  |
  |____reply C

I'm trying to write a web based newsreader using PHP, and can connect and
retrieve articles to display in an article list. However I'm unsure how I
can figure out the threading structure? Is it possible to get this
information from the Headers? If so, which one?

Any advice appreciated,

Thanks

Paul