Warning for mysql_fetch_assoc() in php

Below is the Warning message getting:

Warning: mysql_fetch_assoc() expects parameter 1 to be resource,
object given in E:\HTML Learn\Hocus\profile.php on line 11

line 11 is :

$get = mysql_fetch_assoc($check);

full code:

            if (isset($_GET['u'])) 
        {
            $username = mysqli_real_escape_string($conn, $_GET['u']);
            if (ctype_alnum($username)) 

            {
                //cek user
                $conn = mysqli_connect("localhost", "root", "") or die("error MYSQL");
                mysqli_select_db($conn, 'skey');
                $check = mysqli_query($conn, "SELECT username, first_name FROM users WHERE username='$username'");
            if (mysqli_num_rows($check)==1) 
            {
                $get = mysql_fetch_assoc($check);
                $username = $get['username'];
                $firstname = $get['first_name'];

            }
            else
            {
                echo "
User tidak terdaftar
";
                exit();
            }
        }
    }
?>