Postgres access using PHP

I am trying to access a Postgres database using PHP from a Web page
and it doesn't work.

Could anyone shed any light on the problem?

This is the error I get - which looks like an authentication problem:-

Unable to connect to PostgreSQL server:  FATAL 1:  SetUserId: user 'root' is not in 'pg_shadow'

What have I overlooked in setting up Postgres for Web access?

--
John