Fatal error: Call to a member function query() on a non-object in db.php on line 23

<?php
require_once "config.php";
 
class Db extends Config {
    private $connection;
 
    function __construct() {
        $this->open_connection();
        echo "OK";
    }
 
    private function open_connection() {
        $this->connection = $mysqli = new mysqli($this->DB_HOST, $this->DB_USER, $this->DB_PASS, $this->DB_NAME);
        $this->connection->query("SET NAMES 'utf8'");
            if ($mysqli->connect_error) {
            die('Connect Error (' . $mysqli->connect_errno . ') ' . $mysqli->connect_error);
 
}
    }
 
 
    public function sql($query) {
    $result = $mysqli->query($query, $this->connection);
    if(!$result) {
        die("Database query failed:" .$mysqli->connect_error);
    }
    return $result;
}
}
$db = new db();
 
?>

<?php
 require_once "../config/db.php"; 
 
 class Mmenu extends Db {
    public function return_menu(){
        $sql = "SELECT id, menu_name FROM pages";
        $res = $this->sql($sql);
        return $res;
    }
 }
 
?>

<?php
require_once "../models/mmenu.php";
 
class Cmenu extends Mmenu {
    function print_menu() {
        $res = $this->return_menu();
        while($row = mysqli_fetch_array($res)) {
            $mname[$row['id']] = $row['menu_name'];
        }
        return $mname;
    }
 
}
 
$var = new Cmenu() ;
$var2 = array();
$var2 = $var->print_menu();
print_r($var2);
 
 
?>

<?php
require_once "config.php";
 
class Db extends Config {
    private $connection;
    public $mysqli;
 
 
 
 
 
    private function open_connection() {
        $this->mysqli = new mysqli($this->DB_HOST, $this->DB_USER, $this->DB_PASS, $this->DB_NAME);
        $this->connection = $this->mysqli
        $this->connection->query("SET NAMES 'utf8'");
            if ($mysqli->connect_error) {
            die('Connect Error (' . $mysqli->connect_errno . ') ' . $mysqli->connect_error);
 
 
 
?>