How to connect a cloud mssql db from xampp?

I want to connect a cloud mssql server from my xampp localhost using pdo? How to do that?

try {
    $hostname = "xxxxxx";
    $dbname = "xxxxxx";
    $username = "xxxxxxx";
    $pw = "xxxxxxx";
    $dbh = new PDO ("mssql:host=$hostname;dbname=$dbname","$username","$pw");
  } catch (PDOException $e) {
    echo "Failed to get DB handle: " . $e->getMessage() . "\n";
    exit;
  }

it gives error Failed to get DB handle: could not find driver

i dont know any other information about the db...only i know is that it is a cloud mssql database...pls help me in this

my localhost php version is 5.6.31 i am using xampp in windows 7