Cannot call MySQL when including class in PHP - php

Question

I've got a problem with include. I'm doing some kind of blog, and at this moment it looks like this:
index.php
article.php
class/art.class.php
Let's focus on article.php, which looks like this:
<?php
$mysqli = new mysqli("","","",""); // here are my connection details
if ($mysqli->connect_error) {
die('Connect Error (' . $mysqli->connect_errno . ') '
. $mysqli->connect_error);
}
$mysqli->query("SET NAMES 'utf8'");
require("class/art.class.php");
$art = new Article();
print_r($art->get_art(trim($_GET['id'])));
$mysqli->close();
?>
And art.class.php is like this:
<?php
class Article {
function get_art($id) {
if(!is_numeric($id)) {
header("Location: index.php");
die("<h2>ID isn't numeric, cannot go on.</h2>'");
}
if($result = $mysqli->query("SELECT * FROM `articles` WHERE id='$id';")) {
while($row = $result->fetch_array(MYSQLI_ASSOC)) {
$art = $row;
}
$result->close();
}
return $art;
}
}
?>
The problem is a response from MySQL. Sorry, I mean no response. And no errors. I figured out that I need to add mysql connection code to class code. But why? How I can connect once to database and call it from anywhere, even from included class?
And sorry if my english is bad..

Answer 1

The get_art function within the Article class does not have access to variables outside of it's scope: please see the answer here.
In order to fix your issue, you may provide access to the $mysqli object by passing it to the constructor of the Article class when you instantiate it:
Article.php:
$mysqli = new mysqli("","","",""); // your connection details
$art = new Article($mysqli);
art.class.php:
class Article {
protected $mysqli;
public function __construct($mysqli) {
$this->$mysqli = $mysqli;
}
function get_art($id) {
// Replace $mysqli with $this->mysqli everywhere you need to
// make database calls
}
}

Answer 2

Although some would recommend that you avoid doing so, you could use PHP's $GLOBALS variable to store your database connection:
$mysqli = new mysqli("","","",""); // here are my connection details
if ($mysqli->connect_error) {
die('Connect Error (' . $mysqli->connect_errno . ') '
. $mysqli->connect_error);
}
$GLOBALS['mysql'] = $mysqli;
This way you would have access to it within your class:
class Article
{
function get_art($id)
{
$mysqli = $GLOBALS['mysqli'];
...
}
}