I'm sorry, I should have explained more clearly.
I am trying to refer to a directory that is password protected from one that is not.
The user will already have authenticated against a mysql database.
My problem is not one of authentication per se, but one of needing to be able to view straight html files, pointed at from a php page.
The php pages already perform all necessary authentication, for the php site.
However the problem is to the php script users to be authenticated without entering their login details a second time (i.e no dialog to be popped up).
I hope that makes a bit more sense. Unfortunately I have not found a way to pass the necessary parameters around so that I can provide authentication in this way.