Executing this script generate the error:
Warning: 1 is not a valid PostgreSQL link resource in test.php on line 24
The main problem is that adding a little bit of code to php source to printout the refcount of the resource just after the pg_connect get executed and zend_list_addref get called 
and just before the pg_exec get really executed i obtain the output at the end of the script, please not how refcount magically decrease between the pg_connect and the pg_exec.

The script...
<?php
// 
// Test script to show the bug
//
$db1=pg_connect("dbname=test");
$res1=pg_exec($db1,"SELECT * from tbltest");
echo "1 ".$db1."<br>";

$db=pg_connect("dbname=test");
$res1=pg_exec($db,"SELECT * from tbltest");
echo "2 ".$db."<br>";
pg_close($db);

$db=pg_connect("dbname=test");
$res1=pg_exec($db,"SELECT * from tbltest");
echo "3 ".$db."<br>";
pg_close($db);

$db=pg_connect("dbname=test");
$res1=pg_exec($db,"SELECT * from tbltest");
echo "4 ".$db."<br>";
pg_close($db);

pg_close($db1);

?>

The output:

Warning: Refcount 2 in /usr/local/httpd/htdocs/test.php on line 6
1 Resource id #1
Warning: Refcount 3 in /usr/local/httpd/htdocs/test.php on line 9
Warning: Refcount 3 in /usr/local/httpd/htdocs/test.php on line 10
2 Resource id #1
Warning: Refcount 3 in /usr/local/httpd/htdocs/test.php on line 14
Warning: Refcount 3 in /usr/local/httpd/htdocs/test.php on line 15
3 Resource id #1
Warning: Refcount 3 in /usr/local/httpd/htdocs/test.php on line 19
Warning: Refcount 2 in /usr/local/httpd/htdocs/test.php on line 20
4 Resource id #1
Warning: 1 is not a valid PostgreSQL link resource in
   /usr/local/httpd/htdocs/test.php on line 24