Start work with PHP OOP - php

Question

I now learn PHP OOP and I want to get yours tips. I create DB connection class, is this okay?
How use "Database" class in another class? Always use "extends"?
Thanks
<?php
//Config
$db_user = 'root';
$db_pass = '';
$db_host = 'localhost';
$db_name = 'test';
class Database
{
private $Database;
private static $instance;
public static function instance()
{
if ( !self::$instance )
self::$instance = new Database();
return self::$instance;
}
public function connect($host, $user, $password, $name)
{
$this->db_link = mysql_connect($host, $user, $password);
mysql_set_charset('utf8');
mysql_select_db($name, $this->db_link);
}
public function query($query)
{
$sql = mysql_query($query);
$row = mysql_fetch_array($sql);
return $row;
}
}
class Book extends Database
{
public function getData2()
{
$sql = $this->query('SELECT * FROM users WHERE price = "7"');
return $sql['name'];
}
}
$db = Database::instance();
$db->connect($db_host, $db_user, $db_pass, $db_name);
$b = new Book();
$res = $b->getData2();
print_r($res);
?>

Answer 1

You can extends the database class but it is not what I suggest.
You could also use the keyword global inside the function where you actually need database to let the function get the database instance from outside; but still some people get nervous about global keyword.
You could pass the instance of the database class as argument of constructor of the class where you need database, in this way you will be able to call db methods with a simple chain: $this->db->connect();

Answer 2

As björn states, a book is not a database, wouldn't recommend extend.
One approach would be to on object initialization of book, pass the reference to the db-layer/object..
$myDb = new dbLayer($settings);
$myBook = new book($myDb);
$a = $myBook->getAllData($someParameter);
in the constructor of book you save the reference to the dblayer...
class book {
var $dbTier;
__constructor($db) {
$this->dbTier = $db;
...
regards,
//t

Answer 3

You can do something like this, this should be easy to understand
class User{
private $db;
public function __construct($db){
$this->db = $db;
}
}
The $db in the constructor parameter would be your database connection and required fom another file. All any how you get to it but not in the class.
require 'file from where you have your database';
$user = new User($db);
Anytime you like to user the database to make a query, you just reference it like below
$this->db->query();

Answer 4

You would not use extends here ideally unless Book is a subclass of Database. You would do something like:
class Book
{
var $db;
function __construct() {
$this->db = Database::instance();
}
}
Then the book class would use the instance of the database object and you can always access it with $this->db inside Book.