PHP - PDO SQLSRV - connection string invalid [87]

I am trying to connect to SQL Server 2008 R2 via PHP. I used the following code:

try {
    $server = "servername.do.ma.in\\MSSQLSERVER";
    $user = "username";
    $pass = "password";
    $db = "databasename";

    $dbh= new PDO('sqlsrv:Server = ' . $server . '; Database = ' . $db, $user, $pass);

    $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch (PDOException $e) {
    echo 'Verbindung fehlgeschlagen: ' . $e->getMessage();
}

      

        
        
        
      

    

But when I run this code, I get the following error:

 SQLSTATE[08001]: [Microsoft][ODBC Driver 11 for SQL Server]SQL Server Network Interfaces: Connection string is not valid [87]. 

      

        
        
        
      

    

Does anyone know this error and know what to do?

Thanks in advance.

Decision:

The port is not defined. The following code worked for me. I added port 1433 to the $ server variable.

$server = "servername.do.ma.in, 1433";
    $user = "username";
    $pass = "password";
    $db = "databasename";

    $dbh = new PDO("sqlsrv:Server={$server};Database={$db};", $user, $pass);