Method name must be a string fatal error class

How can I fix this code? When I run check.php I get this error:

Erorr : Fatal error: Method name must be a string in C:\AppServ\www\Weboo\cms\check.php on line 46

<?php
    ob_start();
    session_start();
    class mysql {


        private $localhost = "localhost";
        private $db_user = "root";
        private $db_pass = "root";
        private $db_name ="webocms";


        function __construct(){

        mysql_connect($this-> localhost,$this->db_user,$this->db_pass);
        mysql_select_db($this->db_name);
        }

    }

    function sql(){

        $username = $_POST ['username'];
        $password = $_POST['password'];

        $sql = "SELECT * from users WHERE username='$username'
         AND password='$password'";

         $query = mysql_query($sql);

         $num = mysql_num_rows($query);

         if ($num > 1){

            $_SESSION['username'] = "username";
            $_SESSION['password'] = "password";

            header("Location: admin/admin.php");
         }else {

            echo "<h2><b> No Users </h2></b>";
         }
    }

    $use=new mysql;
    $use->$sql();
    ob_end_flush();
    ?>