psycopg : web service

Dear all,

I would like code a web service with python. I have already imported
several vector data (land cover) and one Digital Elevation Model
(raster layer) into my postgresql/postgis database (server side).

I succeed in connecting to my pg db via psycopg2.

Client side, operators use a client application (Developed with PHP /
javascript / openlayers).

Objectives : Client side, once the layer would be selected,and once
the operators have clicked on the map , they would like that usefull
informations appear on the interface of the client application(kind of
land cover, z of the DEM).

=> Regarding my python script, i have to type a SQL query in order to
select usefull informations of the db layers. And, of course, the
information must depend on geographic coordinates (Latitude
Y/Longitute X).
In a 2nd time, my script must produce a result(JSon type) for the
client side.

It would be possible to help me regarding the coding in order to
select usefull informations of the db layers. (depends on geographic
coordinates x/y ? Could you throw light for me regarding the way to
do this SQL query ?

In advance, thank you to throw light for me.