user and DB confusion

I created a new user and then a new DB with that user, where the user
is the owner:

-bash-3.00$ createuser -d -P
Enter name of user to add: arnuld
Enter password for new user:
Enter it again:
Shall the new user be allowed to create more new users? (y/n) n
CREATE USER

[arnuld(at)dune ~]$ createdb -O arnuld -U arnuld arnuldforum
CREATE DATABASE

when I pass this username "arnuld" and the password for "arnuld" to
phpBB3 (for installation) it says "could not connect to database"
while I can connect to database without any problem. I don't
understand why phpbb says there is something wrong with DB settings

[arnuld(at)dune ~]$ psql arnuldforum
Welcome to psql 7.4.17, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit

arnuldforum=>

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