Help, i'm really desperated

Hi,

I need to run this command inside a PHP (web) script, in order to
produce an acl list (url_negados.txt) for squid:

/usr/bin/psql -t knet -c "SELECT url_negado from urls_negados" >
/tmp/urls_negados.txt

I'm using shell_exec to run it.

Well, so far, so good.
The problem is that i must run this command as user postgres, otherwise
it won't work and i can't find a way to do it in PHP :(

Any ideas would be appreciated.
I'm realy desperated.

Warm Regards,
Mário Gamito