Why not global $db in model classes? - php

Question

Let's say that we have a MVC pattern. If we have something like this:
blog_controller.php
<?php
class c_blog{
private $model;
public function __construct(){
include( [model_path] );
$this->model = new m_blog();
}
public function post_list(){
return $this->model->get_list();
}
}
blog_model.php
<?php
class m_blog{
public function get_list(){
global $db;
$sel = $db->query("...");
$sel = $db->fetch_array($sel);
return $sel;
}
}
index.php
<?php
$db = new database();
include "blog_controller.php";
$blog = new c_blog();
var_dump( $blog->post_list() );
?>
Why the above example isn't good, and a lot of devs want OOP like class m_blog extends database{} ?
I want only the advantages and disavantages, because I use this "worry" method on over 15 sites with a high traffic and I don't have problems (some of that websites work with multidatabases and it's very easy to change global $db, to global $db_five.)
Thanks!

Answer 1

Why not use inheritance?
<?php
class model {
private $db;
public function __construct() {
$this->initDb();
}
private function initDb();
}
class c_blog {
private $model;
public function __construct(){
include( [model_path] );
$this->model = new m_blog();
}
public function post_list(){
return $this->model->get_list();
}
}
class m_blog extends model{
public function __construct() {
parent::__construct;
}
public function get_list(){
$sel = $this->db->query("...");
$sel = $this->db->fetch_array($sel);
return $sel;
}
}