Strange behaviour - differences between http://localhost and http://servername/ (on IIS)

Hello together,

the SQL Server Driver for PHP drives me crazy. I've got a Windows Server 2003 and a IIS6.0 running on it.

My Server connects to a SQL Server 2005. I've got the Native Client 2008 installed on my IIS Server. I connect to the server via sqlsrv_connect('SERVERNAME',array('DB'=>'mygreatdb'));

The code should use the worker process domain account to connect to the sql server. This works actually, but only when I'm on the IIS server via Remotedesktop and open the webpage as http://localhost/.

When I open the page on my client computer via http://servername/ I'm getting the following sqlsrv_error Array:

Array
(
  [0] => Array
    (
      [0] => 28000
      [SQLSTATE] => 28000
      [1] => 18456
      [code] => 18456
      [2] => [Microsoft][SQL Server Native Client 10.0][SQL Server]Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON'.
      [message] => [Microsoft][SQL Server Native Client 10.0][SQL Server]Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON'.
    )

  [1] => Array
    (
      [0] => 28000
      [SQLSTATE] => 28000
      [1] => 18456
      [code] => 18456
      [2] => [Microsoft][SQL Server Native Client 10.0][SQL Server]Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON'.
      [message] => [Microsoft][SQL Server Native Client 10.0][SQL Server]Login failed for user 'NT AUTHORITY\ANONYMOUS LOGON'.
    )

)

I don't know why this happens. It's so strange. I tried to find a configuration tool for the native client or some configuration option in the driver for php, but I couldn't find anything.

I'd be very happy if anyone could help me somehow to fix this error.

Greetings

-Thomas