Well I looked a litle closer at phpPgAdmin on my shared hosting server and saw that it shows entries for:
Server Host Port Username Actions

the server entry is set to PostgreSQL
the Host entry is empty
the Port entry is set to 5432

so it would seem that phpPgAdmin also uses a blank for the ...

Your PHP script uses socket authentication, whereas Serendipity needs to use TCP authentication to be compatible to the usual PGSql workage.

So you must edit your pg_hba.conf to enable TCP/IP authentication. It's basically a PGSql configuration issue, not a Serendipity/PHP problem. You might get ...