$con=mysqli_connect("localhost","my_user","my_password","my_db");
// Check connection
if (mysqli_connect_errno()){
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$username = $_REQUEST['username'];
$password = $_REQUEST['password'];

$tsd=mysqli_query($con, "SELECT * FROM `admin` WHERE `username` = '$username' AND `password` ='$password'")or die("Problem with the query");