$stmt = $mysqli -> prepare('SELECT username FROM users_table WHERE username = ?');
$stmt -> bind_param("s",$username);
$stmt -> execute();
$stmt -> bind_result($result);
$stmt -> fetch();
if ($result==NULL)
{
    $error = "User " . $username . " does not exist!";
    include 'form.html.php';
}
else
{
    $stmt = $mysqli -> prepare('SELECT username FROM users_table WHERE username = ? AND password=MD5(?)');
    $stmt -> bind_param("ss",$username,$password);
    $stmt -> execute();
    $stmt -> bind_result($result);
    $stmt -> fetch();

    if ($result==NULL)
    {
        $error = "Password for user " . $username . " is incorrect!";
        include 'form.html.php';
    }
    else
    {
        echo "Welcome " . $result . "!!";
    }
}