php - mysql - Fatal error: Call to a member function -

trying create login screen , error

fatal error: call member function prepare() on non-object in c:\xampp\htdocs\login\classes\mysql.php on line 19

    <?php   require_once 'includes/constants.php';  class mysql {     private $conn;      function __construct() {         $this->conn = new mysqli (db_server, db_user, db_password, db_name) or die('there problem connecting database.');       }      function verify_username_and_pass($un, $pwd) {          $query = "select *                     users                     username = ? , password = ?                     limit 1";          if($stmt = $this->conn->prepare($query)) {             $stmt->bind_param('ss', $un, $pwd);             $stmt->execute();              if($stmt->fetch()) {                 $stmt->close();                 return true;             }         }     } } 

constants.php

<?php // define constants here  define('db_server', '192.168.2.50:3306'); define('db_user', 'user'); define('db_password', 'pass'); define('db_name', 'membership'); 

there's no object 'mysql' in php. try mysqli