MyProject/app/models/model_user.php

<?php

/**
 * @property $db
 */
class Model_user extends Model
{
    private $db;
    public function __construct()
    {
        $this->db = new db();
    }

    public function register($login, $password){
        // создаем пользователя в БД

        $this->db->query("INSERT INTO users SET `login` = " . $this->db->escape($login) .
            ", `password` = " . $this->db->escape($password)
        );
    }

    public function get_user_by_login($login){
        return $this->db->query("SELECT * FROM users WHERE `login` = " . $this->db->escape($login));
    }
}