I finally did it with ADODB.
There you can change the Codepage for the connection:
$dsn = "DRIVER={SQL Server};SERVER=$dbhostname;DATABASE=$dbname;UID=$dbuser;PWD=$dbpassword";
$conn = new COM("ADODB.Connection",NULL,CP_UTF8) or die("Cannot start ADO");
Thank you,jpadie, for your help,
Kai