$db = mysqli_conect("localhost", "root", "password", "testsite");
$user = $_POST['username'];
$salt = "SELECT salt FROM users WHERE username = $user";
$saltquery = mysqli_query($db, $salt);
$pass = md5(sha1(trim($_POST['password'])) . $salt);
$checkpass = "SELECT password FROM users WHERE username = $user";
$cpquery = mysqli_query($db, $checkpass);
if($pass == $cpquery)
{
    //Code to execute if login is successful
}
else
    //Code to execute if login is incorrect
}