มีปัญหารายการตระกร้าสินค้ามีข้อความเตือน warning ครับ

<?php
$con=mysqli_connect("localhost","my_user","my_password","my_db");
// Check connection
if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

// Change character set to utf8
mysqli_set_charset($con,"utf8");
return $con;
mysqli_close($con);
?>

<?php
class my_db{
	private $db;
	public function __construct( ){
		$this->db=new mysqli('localhost', 'user', 'psw', 'dbname') or die(mysqli_connect_error());
		$this->db->set_charset('utf8');
	}
	public function readProductById($productId){
		$result = $this->db->query("select * from products where id =$productId");
		if( $result->num_rows>0){
			return $result->fetch_assoc();
		} return false;
	}
}
$mydb = new my_db();
$product = $mydb->readProductById(1);