Simple PHP OOP and MySQLI - php

Question

I am trying to learn some more PHP, OOP and MySQLI, but I am stuck. Google didn't help me and searching on this website didn't give me any results either. I have a config class to connect to the database, and in another class I want to run some queries, but I am getting this error with whatever I am trying:
Fatal error: Call to a member function query() on a non-object in
test.php on line 9
Can someone please help me?
config.php:
<?php
class Database
{
private $host;
private $user;
private $password;
private $db;
private $mysqli;
function __construct() {
$this->host = "private";
$this->user = "private";
$this->pass = "private";
$this->data = "private";
$this->mysqli = new mysqli($this->host, $this->user, $this->pass, $this->data);
}
public function query($query)
{
return $this->mysqli->query($query);
}
}
?>
test.php:
<?php
class Dummy
{
private $Database;
function __construct()
{
$this->Database = new Database();
$this->Database->test = $this->mysqli->query("SELECT test FROM test")->fetch_object()->test;
}
}
?>
index.php:
<?php
require 'config.php';
require 'test.php';
$test = new Database();
$test = new Dummy();
echo $this->Database->test;
?>

Answer 1

How about using Dependency injection, create the Database object then pass that as a parameter to the class through the construct.
<?php
class Database
{
private $host;
private $user;
private $password;
private $db;
private $mysqli;
function __construct($host,$user,$pass,$data) {
$this->host = $host;
$this->user = $user;
$this->pass = $pass;
$this->data = $data;
$this->mysqli = new mysqli($this->host, $this->user, $this->pass, $this->data);
}
public function query($query)
{
return $this->mysqli->query($query);
}
}
?>
...
<?php
class Dummy
{
function __construct(Database $db)
{
$this->db = $db;
}
function get_test_yada(){
return $this->db->mysqli->query("SELECT test FROM test")->fetch_object()->test;
}
}
?>
...
<?php
$db = new Database('127.0.0.1','root','pass','database');
$dummy = new Dummy($db);
$dummy->get_test_yada();
?>
Untested, hope it helps

Answer 2

For the goodness sake, DON'T learn them all at once. You won't get any sensible result.
Each is a hard and complex topic alone, of which OOP is hardest.
For the database interacton use PDO prepared statements.
As the OOP practice, never extend class from a service, but use it as a class variable.
class Dummy
{
protected $db;
function __construct($pdo)
{
$this->db = $pdo;
}
}

Answer 3

$this->mysqli = new mysqli($this->host, $this->user, $this->pass, $this->data);
If connecting fails, you can't call anything on that member variable. Check if your connection is established first.
And, if you want to have access to that variable, you should make it public.