Socket Connection Authentication

Hello,
I try to use PHP with Postgresq1 socket connection. All works fine with
following entry in

pg_hba.conf

local      database  user trust
or
local       database  user peer map=someuser


pg_ident.conf

# MAPNAME       SYSTEM-USERNAME         PG-USERNAME
someuser        www-data                db-user

When I try

local      database  user md5

I get the error: "Peer authentication failed for user ..."
Is it possible to authenticate a user with md5 via socket connection?