<form action="zaloguj.php" method="post">
    <label for="login">Login: </label><input type="text" name="login" />
    <label for="password">Hasło: </label><input type="text" name="password" />
    <input type="submit" value="zaloguj" />
</form>

<?php
interface storage {
    public function login($name, $pass);//1
}
class mysqlStorage implements storage{//2
    public function login($name, $pass){//3
        global $pdo;
        $pdo=new PDO('mysql:host=localhost;dbname=nazwabazy', 'root', 'root');
        $stmt=$pdo->prepare('SELECT name, ranga FROM `users` WHERE name=:name AND password=:password');
        $stmt->bindValue(':name', $name);
        $stmt->bindValue(':password', $pass);
        $stmt->execute();
        $user=$stmt->fetch();
        if($user){
            return $user;
        }
        return false;
    }// login();
}//mysqlStorage
class user{//4
    public $name;
    public $rank;

    private $storage;
    public function __construct($storage){
         $this->storage=$storage;
         }
    public function zaloguj(){
         if($_SERVER['REQUEST_METHOD']=='POST'){
             $user=$this->storage->login($_POST['login'], $_POST['password']);
             if(!user){die('Nieprawidłowa nazwa użytkownika lub hasło!');}
             $this->name=$user['name'];
             $this->rank=$user['ranga'];
             $_SESSION['zalogowany']=true;
             $_SESSION['name']=$this->name;
             $_SESSION['rank']=$this->rank;
             }
         }
    $user=new user(new mysqlStorage);
    $user->zaloguj()
}
?>

<?php
public function rejestracja($name, $password, $email, $regdate);
?>

<?php
public function rejestracja($name, $password, $email, $regdate){
    $stmt=$pdo->prepare('INSERT INTO `users` VALUES(NULL, :name, :password, :email, :regdate, 2)');
    $stmt->bindValue(':name', $name);
    $stmt->bindValue(':password', md5($password));
    $stmt->bindValue(':email', $email);
    $stmt->bindValue(':regdate', $regdate);
    $stmt->execute();
    }
?>

<?php
public function register(){
     $this->storage->rejestracja($_POST['name'], $_POST['password'], $_POST['email'], date('d-m-Y'));
     }
?>