PHP SQLSRV connection “Failed to get DB handle: could not find driver”

I'm using the following code:

try {
    $hostname = "*****";
    $port = 1443;
    $dbname = "*******";
    $username = "********";
    $pw = "************";
    $dbh = new PDO ("dblib:host=$hostname:$port;dbname=$dbname","$username","$pw");
} catch (PDOException $e) {
    echo "Failed to get DB handle: " . $e->getMessage() . "\n";
    exit;
}

Error:

Failed to get DB handle: could not find driver

The DLL Files are already in the php/ext and modified in the php.ini

What else can I do?