web control needed - suggestions?

Folks,

I have need of something like a multilayer combo box to allow
selection (or addition) of hierarchical data on a web site. I dont
really know how to achieve this and so am hoping for suggestions as to
how to go about it. If it can be achieved somehow using
css,js,perl,php would be great as I have all those available to me.
Otherwise, any suggestions of how to do it using alternative methods
would be greatly appreciated.

An example of the data I have is structured as follows:-

Continent Country Province City
========= ======= ======= ===========
North America USA California Los Angeles
North America USA California Sacremento
Australaisa Australia NSW Sydney
Australaisa Australia NSW Newcastle


Ideally i would like a control like a combo box which allows you to
add a value to any level of the hierarchy if the one you want is not
in the current selection list.

So say you lived in cantong - china and wanted to add your location
you would drop down the location box, see it contained only 2
continents and therefore type in asia instead of making a selection.
Then upon dropping down the box again Asia would now show in the list.
When the user mouseovers the entry "asia" in the continents list, the
second layer of the hierarchy (country) would show to the right. It
would currently have no entries but show an imput box to allow adding
a new entry. The customer would add "China".
And so on down the location tree until they had chosen (or added)
their own city.

I realise this is quite specialised, and am open to alternative ways
to achieve the same thing. However I ideally would like a single
control with configurable number of hierarchical layers.

Dont want much, do I ;-)

Regards