Warning: mysql_fetch_assoc() expects parameter 1 to be resource, string given in Z:\... on line 24, в чем проблема?

Не могу понять в чем ошибка, вот код:
<?php

$servername = "localhost";
$username = "root";
$password = "";

$conn = mysql_connect ($servername, $username, $password)
or die("Нет соединения: " . mysql_error());

echo "
\n";

mysql_select_db("hismatov_bd", $conn);

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

$r = "select * from users where password = $password and login = $login";
$result = mysql_fetch_assoc($r);
$user = $result[0];
?>