pg_connect

It is worth to know, that you can set application_name in connection string, consider this simple example:
<?php
$appName = $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
$connStr = "host=localhost port=5432 dbname=postgres user=postgres options='--application_name=$appName'";
//simple check
$conn = pg_connect($connStr);
$result = pg_query($conn, "select * from pg_stat_activity");
var_dump(pg_fetch_all($result));
?>

By doing this move on cli or cgi you can see in pgAdmin what scripts are running or what requests are running on database. You can extend configuration of postgres to track slow queries and print application name to logs. It was very usuful to me to find out what and where should I optimize.

If you use pgbouncer and unix socket
and you pgbouncer.ini looks like this
listen_port = 6432
unix_socket_dir = /tmp
you connect like this
pg_connect('host=/tmp port=6432 dbname=DB user=USER password=PASS');

Getting md5 passwords was confusing because of a lack of documentation:
- set up your pg_hba.conf in order to use md5 password instead of 'trust' or 'ident'
- check if your postgres.conf has 'password_encryption=on' (depending on the version this might already be 'on').
- make sure to restart your postgres process.
- in PHP you just supply the username and password in _plain_ text:
'host=localhost port=5432 dbname=megadb user=megauser password=holyhandbagsbatmanthispasswordisinplaintext'
The postgres PHP library will automagically do the md5 encoding for you, no need to do it yourself.

Beware about writing something like 
<?php
function getdb_FAILS() {
 return pg_connect("...") or die('connection failed');
}
?>

It will return a boolean. This will appear to be fine if you don't use the return value as a db connection handle, but will fail if you do.
Instead, use:
<?php
function getdb() {
 $db = pg_connect("...") or die('connection failed');
 return $db;
}
?>

which actually returns a handle.

It's not explicitly stated here, but you can also connect to PostgreSQL via a UNIX domain socket by leaving the host empty. This should have less overhead than using TCP e.g.:
$dbh = new PDO('pgsql:user=exampleuser dbname=exampledb password=examplepass');
In fact as the C library call PQconnectdb underlies this implementation, you can supply anything that this library call would take - the "pgsql:" prefix gets stripped off before PQconnectdb is called, and if you supply any of the optional arguments (e.g. user), then these arguments will be added to the string that you supplied... Check the docs for your relevant PostgreSQL client library: e.g.
http://www.postgresql.org/docs/8.3/static/libpq-connect.html
If you really want, you can use ';'s to separate your arguments - these will just be converted to spaces before PQconnectdb is called.
Tim.

At least with Postgres 7.2, connecting to local postgresdatabase requires a user in the database with the same name as the user running apache, or the connection fails.

One thing is to remember, whenever trying to use pg_connect, add the timeout parameter with it
<?php
$d=pg_connect('host=example.com user=pgsql dbname=postgres connect_timeout=5');
?>

For what it's worth, it should be noted that, while PHP will generally handle connection-reuse for you so long as you keep using the same connection strings, as in the following example:
<?php
$before_conn1 = microtime(true);
$db1 = pg_connect($conn_string);
$before_conn2 = microtime(true);
$db2 = pg_connect($conn_string);
$after_conn2 = microtime(true);
echo($before_conn2 - $before_conn1); // Takes ~0.03s
echo("\n");
echo($after_conn2 - $before_conn2); // Takes 0s
?>

...as nice as it would have been, this does not hold true for async connections; you have to manage those yourself and you can't follow up an async connection with a blocking one later on as an easy way to wait for the connection process to complete before sending queries.
<?php
$before_conn1 = microtime(true);
$db1 = pg_connect($conn_string, PGSQL_CONNECT_ASYNC);
sleep(1);
$before_conn2 = microtime(true);
$db2 = pg_connect($conn_string);
$after_conn2 = microtime(true);
echo($before_conn2 - $before_conn1); // Takes ~1s
echo("\n");
echo($after_conn2 - $before_conn2); // Takes ~0.025s
?>

It's possible connect to a PostgreSQL database via Unix socket using the pg_connect() function by the following two ways:
1) Using the socket path:
<?php
$conn = pg_connect('host=/var/run/postgresql user=username dbname=databasename');
?>

2) Omitting the host name/path:
<?php
$conn = pg_connect('user=username dbname=databasename');
?>

Note: in this case (omitting the host value), the default socket path will be used.

The values accepted by pg_connect's sslmode argument are: disable, allow, prefer, require

If you use PostgreSQL users for authenticating into your pg database rather than using your own authentication, always specify host directive in pg_connect and edit pg_hba.conf to authenticate from this host accordingly. Otherwise, PHP will connect as 'local' using UNIX domain sockets, which is set in pg_hba.conf to 'trust' by default (so you can connect using psql on console without specifying password) and everyone can connect to db _without password_ .

I got the same problem but I have to solve that in different way.
In my postgresql.conf file the following was commented.
So, I active that under Connection Settings-
# - Connection Settings –
tcpip_socket = true

Using the "service" parameter as the connection string -- we found that the following functions:-
putenv("PGSERVICEFILE=/path/to/your/service/file/pg_service.conf");
$connect_string = ("service=testdb");
try {
 $pgconn_handle = pg_connect($connect_string);
 . . . . . etc.
Note:-
1) the environment variable has to point to the path AND file name.
2) the file has to be readable by Apache.
See:-
https://www.postgresql.org/docs/9.6/static/libpq-pgservice.html
for how to create your pg_service.conf

regarding the note from matias at nospam dot projectcast dot com
on 12-Feb-2002 01:16, you do not need a user in the database with the same name a your web user with ANY version of postgresql. The only time that would be a requirement ifs if you set your postgresql server to only allow IDENT based authentication (which IIRC is the default on Red Hat systems, which might be what lead to the confusion). For more info on the various authentication methods allowed by postgresql, check out http://www.postgresql.org/docs/7.4/static/client-authentication.html 

If you use host=HOSTNAME in your pg_connect string when connecting to PostgreSQL databases newer than 7.1, you need to make sure that your postmaster daemon is started with the "-i" option. Otherwise the connection will fail. See http://www.postgresql.org/idocs/index.php?client-authentication.html for client authentication documentation.

if you need to open a new connection handle (i.e. for multiple pg_send_query()) use PGSQL_CONNECT_FORCE_NEW as second parameter to pg_connect()

Little note that is buried in the install somewhere. In Php 3, PostgreSQL support was activated by adding --with-postgresql=[DIR] to the options passed to ./configure. With Php 4.0.2 (on Linux) the parameter was --with-pgsql. The only place I found this was in the installing PHP on Unix section of the manual.

pg_connect() won't work with the authentication method 'crypt' in the pg_hba.conf. Took me an hour to figure that out till I remeberd some other issues with windows missing the crypt() call.