server IP : 192.168.137.4
Windows Authentication : Windows Authentication
UserName : DELL-M102Zdell
Database : DataProd
Network Protocol : <default>
Product Name : Microsoft SQL Server Express Edition
Server Name : DELL-M102ZSQLEXPRESS
Instance Name : SQLEXPRESS
Computer Name : DELL-M102Z

$serverName = "DELL-M102ZSQLEXPRESS"; //serverNameinstanceName
$username = "DELL-M102Zdell"; //serverNameinstanceName
$conn = mssql_connect( $serverName,$username,'');

1. make sure you have allow network connection from sql server configuration tool
2. allow connection for this port in firewall
3. activate sql browser service
4. make sure port is listen as the the service provide

<?php
$myServer = 'xxx.xxx.xxx.xxx:yyyy';
$myUser = 'sa';
$myPass = 'xxxxx';

$con = mssql_connect($myServer, $myUser, $myPass) or die("Could not connect to database: ".mssql_get_last_message());
if($con){
echo "connected";
}
// Select a database:
mssql_select_db('new')
    or die('Could not select a database.');

// Example query: (TOP 10 equal LIMIT 0,10 in MySQL)
$SQL = "SELECT TOP 10 * FROM Table";

// Execute query:
$result = mssql_query($SQL)
    or die('A error occured: ' . mysql_error());

// Get result count:
$count = mssql_num_rows($result);
print "Showing $count rows:<hr/>nn";

// Fetch rows:
while ($Row = mssql_fetch_assoc($result)) {

    print $Row['BillNo'] . "n";

}

mssql_close($con);