Почему предупреждение: mysqli_query (): Не удалось получить mysqli?

Я не понимаю, почему я удаляю код в строке 14 ($ this-> close ();), у него нет ошибки, но я не удаляю его, а затем предупреждает mysqli_query (): Не удалось получить mysqli. Это в конце конструкции ??? Моя ошибка: введите описание изображения здесь Мой код:

<?php class Display extends Awebarts { private $conn; private $tablename; public function __construct($tablename) { $this->tablename = $tablename; $this->connectToDb(); $this->conn = $this->getConn(); // insert the data into the table $this->getData(); $this->close(); } function getData() { $query = "SELECT * FROM $this->tablename ORDER BY `id` DESC LIMIT 1 "; if(!$sql = mysqli_query($this->conn, $query)) { throw new Exception("Error: Can not excute the query."); } else { $num = mysqli_num_rows($sql); while($num > 0) { //var_dump($data); $data = mysqli_fetch_array($sql); $num--; } } return $data; } } class Awebarts { private $cxn; private $conn; function connectToDb() { include "models/Database.php"; $vars = "include/vars.php"; $this->cxn = new Database($vars); $this->conn = $this->cxn->getConn(); } function getConn() { return $this->conn; } function close() { $this->cxn->close(); } } class Database { private $host; private $user; private $password; private $database; private $conn; function __construct($filename) { if(is_file($filename)) { include $filename; } else { throw new Exception("Error"); } $this->host = $host; $this->user = $user; $this->password = $password; $this->database = $database; $this->connect(); $this->selectData(); } function getConn() { return $this->conn; } private function connect() { // connect to the sercer if(!mysqli_connect($this->host, $this->user, $this->password)) { throw new Exception("Error: not connected to the server"); } else { $this->conn = mysqli_connect($this->host, $this->user, $this->password); } return $this->conn; } private function selectData() { if(!mysqli_select_db($this->conn, $this->database)) { throw new Exception("Error: No database"); } } function close() { mysqli_close($this->conn); } } ?>