foo@bar:~$ printf 'SELECT * FROM users' | psql -U foo foo
 user_id |       username       | mail_host | alias | language | preferences
---------+----------------------+-----------+-------+----------+-------------
      34 | testuser@terrist.org | localhost |       | en_US    |
(1 row)

foo@bar:~$ cat /tmp/test.php 
#!/usr/bin/php
<?php
require_once 'MDB2.php';
$dsn = 'pgsql://foo:@/foo';
$dbh =& MDB2::connect($dsn);

if (PEAR::isError($dbh)) {
  echo ($dbh->getMessage().' - '.$dbh->getUserinfo());
}
$query = 'SELECT * FROM users';

$result = $dbh->query($query);

// check if the query was executed properly
if (PEAR::isError($result)) {
  echo ($result->getMessage().' - '.$result->getUserinfo());
  exit();
}

// lets just get row:0 and free the result
$array = $result->fetchRow(MDB2_FETCHMODE_ASSOC);
var_dump(array('row' => $array));

$result->free();
$dbh->disconnect();
foo@bar:~$ /tmp/test.php 
array(1) {
  ["row"]=>
  NULL
}
foo@bar:~$