Postgresql, number of connections and life time

Hello,

First of all sorry about my bad command of english.

I have two questions about postgres.

First.
I have a Linux box with postgres 7.4 branche, where web php applications
queries the DB. When Apache launches a php file that connects to the DB,
if I do "ps aux | grep postgrest", I can see a process "postgres user
...." running, that represents the session openned to the DB. But when
the php program finishes, and closes the connection, this process does
not end. Therefore the number of processes are growing while people
visit the web. ¿ Why ? I see the php code, and there is a closing
command before end.
I see that sometimes I visit a page that connects to the DB and no new
process is performed or remains. I also realise that after several hours
(for example at midnight) those processes don't exist.
What is happening ? Is it usual ? Is there a param to control the number
of process launched and their life times?
In PHP I don't use a pool of connections because, the problem is the
same (there are several processes, and remain after connection closes).

Second.
I gain access this DB remotely by an encrypted connection with stunnel.
Well, when I launch a connection to a DB, and passes a long time without
traffic, I realise the connection is closed. The program that queries
the DB does not realise the connection is broken and when it tries to
query, it fails. I think that the origin of the problem is not stunnel,
because the same problem presents to me when trying with a testing DB
without stunnel. ¿ Why ? Has postgres a timer that breaks the
connections when there is no traffic ? Has Linux this timmer ? Is the
carrying data enterprise who closes the connection ? Is this a TCP/IP
stack protocol question ?

Thanks you very much.