MySQLI in classes and in functions - php

Question

My question is rather simple and after checking a few bits and bobs on here I feel its best to ask a new question.
Lets say I have 2 classes
class FirstClass {
function test() {
return "info";
}
}
class SecondClass {
function test() {
return "info";
}
}
Then I have my mysqli object
$mysqli = new mysqli(host, user, password, db);
What do i need to do to be able to use the mysqli object inside the functions inside the classes.
This is my 2 thoughts so far although I haven't placed it on a site for testing yet.
class FirstClass {
global $mysqli;
function test() {
$mysqli->query("some query");
return "info";
}
}
or
class FirstClass {
function test() {
global $mysqli;
$mysqli->query("some query");
return "info";
}
}
I am pretty sure I can setup a construct if need be but I just need to know which way is the best way/only way to share the mysqli object.
Thanks
EDIT:
So I have done a hell of a load of learning and now have a lot more experience with passing info in and out.
Here is a latest working example type that I use.
namespace Page;
use mysqli;
class edit extends details{
protected $db;
//this function is actually in the details class but there is no point in demoing 2 classes
function __construct(mysqli $con){
$this->db = $con;
}
}

Answer 1

To expand what Kneel told you in comments and to counter the other answer
class foo {
function __construct($mysqli){
$this->mysqli = $mysqli;
}
function test() {
return $this->mysqli->query("some query");
}
}
is what it have to be.
You should create a mysqli instance somewhere outside the class and then pass it in coustructor.

Answer 2

You could use __construct() to initialize your MYSQLi. You can then access it around your class with $this.
class FirstClass {
public function __construct(){
$this->mysqli = new mysqli("host", "user", "password", "db");
}
function test() {
$this->mysqli->query("some query");
return "info";
}
}
If you wanted to use it in your second class too, you could construct it in the same way or extend your first class.
class SecondClass extends FirstClass {
public function __construct(){
parent::__construct();
}
function test() {
return "info";
}
}