SQLSTATE[08006] [7] server closed the connection unexpectedly

I do have a strange probleme when connecting to a database thru php using
PDO.
When connecting to the php script thru a browser i get the following error :
HTTP 500 (Internal Server Error)

Not really informative, then i've tried the php script thru Command Line
where i get :
yt(at)D620 /home/yt/Sites/yt_tests $ php index.php
PHP Fatal error: Uncaught exception 'PDOException' with message
'SQLSTATE[08006] [7] server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.' in
/home/yt/Sites/yt_tests/index.php:75
Stack trace:
#0 /home/yt/Sites/yt_tests/index.php(75):
PDO->__construct('pgsql:dbname=yt...', 'yt', 'yvon5533')
#1 {main}
thrown in /home/yt/Sites/yt_tests/index.php on line 75
zsh: exit 255 php index.php

Line 75 being :
$db = new PDO("pgsql:dbname=$dbname;host=$host;", $username, $password );

I should say i do NOT have any prob when connecting directly to this
database using :
$ psql -h <IPV4 or IPV6 of the PostgreSQL server> -U yt -d yt_tests

I'm stuck, i don't see any way to debugg that prob.

--
Yvon