Check what php.ini file is actually loaded. Most likely you're just missing the proper extensions. Then change that script a bit:

<?php

define('DB_HOST','192.168.1.20');
define('DB_NAME','db1');
define('DB_USER','user');
define('DB_PASS','pass');
try {
  $pdo = new PDO('mssql:host='.DB_HOST.'dbname='.DB_NAME,DB_USER,DB_PASS);
}catch (PDOException $e) {
  print $e->getMessage();
}
?>

And post the output here.

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

Very similar problem here with pdo_dblib.  Works fine under Apache module, but not CLI.

I've tried to ensure everything was built correctly.

<?php

$user = 'foo';
$pass = 'bar';
$dbh = new PDO('dblib:host=odyssey:2638;dbname=odyssey', $user, $pass);

?>

> php testpdo.php

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000] Server is unavailable or does not exist. (severity 9)' in /usr/home/kian/public_html/testpdo.php:5
Stack trace:
#0 /usr/home/kian/public_html/testpdo.php(5): PDO->__construct('dblib:host=odys...', 'foo', 'bar')
#1 {main}
  thrown in /usr/home/kian/public_html/testpdo.php on line 5

> php -v
PHP 5.2.6 with Suhosin-Patch 0.9.6.2 (cli) (built: Jul 22 2008 16:30:13) (DEBUG)
Copyright (c) 1997-2008 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2008 Zend Technologies

FreeBSD 7.0.

Same issue with me:
Neither PDO lib is working nor MSSQL

I have tried both  

1) MSSQL: 
try 
		{
			$pdo = new PDO ("mssql:host=".$servername.":".$port.";dbname=".$dbname,$username,$password);
		} 
		catch (PDOException $e)
		{
			echo "Failed to get DB handle: " . $e->getMessage() . "\n";
			exit;
		}

Error is: Failed to get DB handle: could not find driver 
2) PDO-lib

		try 
		{
			$pdo = new PDO ("dblib:host=".$servername.":".$port.";dbname=".$dbname,$username,$password);
		} 
		catch (PDOException $e)
		{
			echo "Failed to get DB handle: " . $e->getMessage() . "\n";
			exit;
		}
Error is: Failed to get DB handle: SQLSTATE[HY000] Server is unavailable or does not exist. (severity 9)

If you are on windows, please try a 5.2 snapshot. (http://windows.php.net/snapshots/)

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".