Most standard PDO tutorials from around the web are going to demonstrate using a MySQL database with apache as a web host. The PDO connection string for this setup is not going to work if you want to connect to a SQL Server database using Microsofts IIS server.
The first step is to make sure PHP is setup to connect to SQL Server. PHP is going to use the sqlsrv driver on IIS in order to connect to SQL Server. This will probably be installed already, but to make sure you will have to go to your php.ini file. If you are unsure where this file is located you can use phpInfo() to find out the file path of the php.ini file.
You need to open the file and check for the following line. If its commented out, uncomment it. If its missing then add it. If it’s not present then there is a chance that the driver is not installed at all, if so then you will need to install it.
If you don’t have the driver installed, you can get it here.
Once you are all setup with the driver and php is good to go, restart IIS to make sure that the changes with the php.ini are picked up by IIS.
When making a query the code stays the same, but the PDO connection string is going to be different. Use the following format and add the connection details for your server into the string.
$db = new PDO ("sqlsrv:server=localhost;database=Database","username","password");
This should be all you need to connect to an SQL Server database using PHP with IIS web host.