Cannot connect to the database (PG 7.3)

Hello people,

I have this Postgres 7.3 database, and could not find if this is a known
issue, but:
I can connect to the database with my postgres user to a remote database.
This as supposed with
psql -d mydb -h 123.456.789.001 -U postgres
it goes fine... postgres user exists on my system and the other system.
Both versions of Postgres are the same (7.3)

However, I have another user, (whose name is a number) that does not exist
on operating system level:
psql -d mydb -h 123.456.789.001 -U 20020003
or from within php:
pg_connect("host=123.456.789.001 dbname=mydb user=20020003 password=secret");

I get this error message:
psql: FATAL: permission denied

while I do not get that message with postgres-user...
Anyone who knows where the problem lies?
I am thinking of:
- Users must exist both in the database and on the operating system. (which
would be strange I think)
- Usernames may not exist solely out of numbers

If the mistake is because I have missed it in the documentation, I am
sorry... I'll try to be more cautious next time...

Michiel