[Proj] conversion latlong to UTM (& vice versa)

[oon at informatika.org]

Dear All Proj user,

I want to convert coordinate with datum WGS84, from latlong projection 
to UTM projection
(and vice versa).

If I convert from latlong projection to UTM projection,
I can get the UTM zone from UTM definition, so I can calculate the zone 
from it's longitude.

If I'm doing vehicle tracking with UTM projection,
and the vehicle moving between UTM zones,
How should I do to get UTM zone of current position of vehicle?
(so I can convert UTM to latlong again).

Is it any statement or definition of UTM that declare
how to get UTM Zone from UTM coordinate?

Is it any Proj utilities (function/procedure) that support my problem?

thanks,

-- 
regards,
Oon Arfiandwi M.