Error in login page in PHP pin

See more:

PHP

Warning : mysqli_prepare() expects parameter 1 to be mysqli, null

What I have tried:

login.php

if(empty($username_err) && empty($password_err)){

global $link;
$sql = "SELECT id, username, password FROM users WHERE username = ?";

error line
if($stmt = mysqli_prepare($link, $sql))


And here is config.php

class dbConfig {
private $serverName = 'localhost';
private $userName = 'root';
private $password = '';
private $dbName = 'dg_db';
protected $conn;



public function __construct()
{

$link = mysqli_connect('localhost','root','','dg_db');

{
try{

$dsn ="mysql:host={$this->serverName}; userName={$this->userName}; charset=utf8";
$options=array(PDO::ATTR_PERSISTENT);
$this->conn = new PDO($dsn, $this->userName, $this->password, $options);

}

catch(PDOException $e)
{
echo "Connection Error: ".$e->getMessage();
}


}
}

Warning : mysqli_prepare() expects parameter 1 to be mysqli, null

I am stuck of this