Set a blank password for a db user

Hi,

I'm new to postgresql and i have an easy question to ask. I have
installed postgresql 8.1 on windows 2000 and i want to use it with my
app developed with php 5 + apache 2.
All works well but i need to know if this is possible :
I use pgAdminIII as administration tool an i need to create a
restriceted grants db user that haven't a password set so i can connect
from php using this statement :

pg_connect ("dbname=dbname user=username password=")

As i saw, if i leave the password blank for the user "username" (inside
the pgAdmin) i cant connect to db from php pages. Instead, if i set a
password for the user and use this statement :

pg_connect ("dbname=dbname user=username password=aaa")

the connection is established ....

Is there a manner to avoid this ?

Thanks in advance,
Claudio