致命错误：未捕获错误：在null（）上调用成员函数query（）

Question

Database.php

// selecting query
     public function select($query){

         And here-> $result = $mysqli->query($query);

        if($result->num_rows > 0){
            return $result;
        }

        else{
            return false;
        }

Header.php

<?php

    include 'libraries/config.php';

include 'libraries/database.php';

//creating new object of database
$db = new database();

$query = "select * from posts order by id desc";

giving error here -> $posts = $db->select($query);

?>

  

致命错误：未捕获错误：在C：\ xampp \ htdocs \ OOP \ libraries \ database.php：40中对成员函数query（）的调用为null：堆栈跟踪：＃0 C：\ xampp \ htdocs \ OOP \ include \ header.php（11）：database-> select（'select * from p ...'）＃1 C：\ xampp \ htdocs \ OOP \ index.php（3）：include（'C：\ xampp \ htdocs ...'）＃2 {main}在第40行的C：\ xampp \ htdocs \ OOP \ libraries \ database.php中抛出