levels in Shapefiles

[Giridhar Manepalli]

Hello All,
I am currently trying few options with php/mapscript to embed in my
application later on.

Say that I have a shapefile for a state with the geometry being
polygon. The polygons inside this shapefile each represent a city
(say). I have also got the shapefiles for each of the city. Now, if I
click on any part of the region inside a city on the main shapefile,
how can I load the shapefile corresponding to the city on which I
clicked.

Is there any standard way to achieve this? Did anyone have an idea
which they think is the standard way?

I appreciate any help in this regard. 

Thanks and Regards,
Giridhar
p.s Currenly I am using ESRI shapefiles. Is any other geometrical
data( for eg Tiger Dataset) more appropriate to do this. What is
google doing for this?(Just curious)