مشکل در کد های php

سلام دوستان طبق آموزش های استاد دارم پیش میرم

دقیقا همون کدی که استاد مینویسه منم مینویسم ولی خطا میده

خط 28 رو خطا میده

 <?php
 class Amin{

private $connection;
private static $db;

public static function getInstance($option = null){
if (self::$db == null){
self::$db = new Amin($option);
}

return self::$db;
}

public function __construct($option = null){
if ($option != null){
$host = $option['host'];
$user = $option['user'];
$pass = $option['pass'];
$name = $option['name'];
} else {
global $config;
$host = $config['db']['host'];
$user = $config['db']['user'];
$pass = $config['db']['pass'];
$name = $config['db']['name'];
}

$this->connection = new mysqli($host, $user, $pass, $name);
if ($this->connection->connect_error) {
echo "Connection failed: " . $this->connection->connect_error;
exit;
}








$this->connection->query("SET NAMES 'utf8'");
}

public function first($sql){
$records = $this->query($sql);
if ($records == null){
return null;
}

return $records[0];
}

public function insert($sql){
$id = $this->connection->query($sql);
return $id;
}

public function query($sql){
$result = $this->connection->query($sql);
$records = array();

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

while($row = $result->fetch_assoc()) {
$records[] = $row;
}

return $records;
}

public function connection(){
return $this->connection;
}

public function close(){
$this->connection->close();
}
}
?>

 ( ! ) Warning: mysqli::mysqli(): in C:\wamp\www\hichkas\amin.php on line 28

این خطارو میده