Using db connection everywhere inside PHP application - php

Question

I have db class which looks like that
class db {
protected $db;
public function __construct() {
$this->connect();
}
protected function connect() {
$this->db = new MySQLi(db_host, db_user, db_pass, db_name) or die($this->db->error);
$this->db->set_charset('utf8');
}
}
Every class inside my PHP app extends this db class
Like
class registration extends db {
var $validation;
function __construct() {
parent::__construct();
$this->validation = new validation();
...
And validation looks like that
class validation extends db {
var $ajax, $common;
function __construct() {
parent::__construct();
...
Getting error message "Too many connections". I feel that, this is not right approach: I'm every time reconnecting to db. So what's right way in your opinion? Is that possible to define('db', ...) 1 time and use everywhere inside app?

Answer 1

registration and validation are classes the use db but are not a sub-class of it.
Your code should look like:
$db = new DB();
$db->connect();
$registration = new Registration($db);
class Registration {
private $db;
public function __construct(DB $db) {
$this->db = $db;
...
You pass a reference to an instance of $db to all classes that require it.
The reason you're opening too many connections is probably because currently each class makes it's own connection to your database, and that is not what you want to do, or need to do.

Answer 2

You want to use composition here instead. Also might consider investigating Singleton pattern.
To elaborate, using composition, each class in your library will have an instance of the db class rather than be an instance of the db class.
Singleton will make the db class enforce only one instance of the class is ever created which is useful for shared resources like database connections. Have a look at this link for further reading on the topic.
http://php.net/manual/en/language.oop5.patterns.php
EDIT: Adding some code
Turning the db class into a Singleton
<?php
class db
{
static private $_oInstance = null;
protected $db;
private function __construct()
{
$this->connect();
}
static public function getInstance()
{
if(self::$_oInstance === null)
self::$_oInstance = new db();
return self::$_oInstance();
}
protected function connect()
{
$this->db = new MySQLi(db_host, db_user, db_pass, db_name) or die($this->db->error);
$this->db->set_charset('utf8');
}
}
Revising the rest of your classes to compose the db instance rather than extend the db class
class registration
{
private $_oDb;
public $validation;
function __construct()
{
parent::__construct();
$this->_oDb = db::getInstance();
$this->validation = new validation();
}
// ...
}