Cannot Select DB using mssql

[newphpcoder]

Good day!

I change my database from mysql to mssql, so I need to change the connection code in my php. I’m not familiar about the codes using mssql. Now cannot connect to my database.
I used the following:
php 5.1.4
IIS 6.0
Microsoft SQL Server 2005 Express Edition
SQL Server Management Studio Express
Here is my code:

<?php
//session_start(); 
//session_regenerate_id(); 

//if($_SESSION['loggedin']){ 
//the user is already logged in, lets redirect them to the other page 
  //  header("Location:company.php"); 
//} 


$server = "ISM\SQLEXPRESS";
//$db = "dbtest";
$connectOptions = array("Database" => "dbtest");
//Because UID and PWD are not specified in the connection option, the connection is made with Windows Authentication.
$conn = mssql_connect($server) or die("Could not connect");


$selected = mssql_select_db($connectOptions)or die("Cannot select DB");    


        $department = $_POST['department'];    
        $username = $_POST['username']; 

        $sql=mssql_query("SELECT `Department`, `Username` FROM `tbluser` WHERE `Department` = '{$department}' AND Username = '{$username}'") or die(mssql_min_error_severity()); 
        $ct = mssql_num_rows($sql); 
      
        if($ct == 1) { 
// im guessing this means that the user is valid. 
$_SESSION['loggedin'] = true; // now that the user is valid we change the session value. 
            $row = mssql_fetch_assoc($sql);   
            
            //$_SESSION['username'] = $row['Username'] ;
            //$_SESSION['department'] = $row['Department'];
            
            $Departments=array('aa', 'bb', 'cc', 'dd', 'ee', 'ff', 'gg', 'hh', 'ii', 'jj');
            
            if (in_array($row['Department'], $Departments)){
                    header ('Location:company.php');
            }else{
                    echo "Incorrect Username or Department";
                    header ('Location:index.php');
            }
        }
?>

And when I run this code the result is:

Cannot select DB

Thank you in advance