Question

I am using simple md5($password); format but i want to add salt so how i can do that?

here is my code :

   if($success)
    {
        $data['firstname'] = $firstname;
        $data['lastname'] = $lastname;
        $data['username'] = $username;
        $data['password'] = md5($password);
        $data['email'] = $email;


        $newUser = new User($data);
        $newUser->save(true);
        $Newuser->login($username, $password);
        header("Location: welcome.php");

    }

Answer 1

$data['hashedpwd'] = md5($salt . $password);

The longer, more complex and unique to each user you can make the salt the harder it will be for anyone to get the password (though it’s not impossible).

A simple (but poor salt) would be: $salt = '10';
A much stronger salt would be: $salt = '-45dfeHK/__yu349@-/klF21-1_\/4JkUP/4';

Salts that are unique to the user are even better.

As mentioned in several comments md5 is an old and relatively poor hashing algorythm, SHA-512 or any of the SHA-2 family would be much better choices.

See this salting question for more detail.