PHP: loginsystem

Hvad er der galt her? Har virkelig prøvet alt, men den vil ikke give noget output, når jeg tester det... Jeg har en session_start i min layout-fil, som jeg inkluderer denne fil i:

<?php

$conn = mysql_connect("localhost", "username", "password");
mysql_select_db("database");

$username = $_REQUEST["username"];
$password = $_REQUEST["password"];

if (empty($username) || empty($password)) {
echo "Remember to fill in your username and password";
}

if (!empty($username) && !empty($password)) {

$query = mysql_query("SELECT * FROM rao_users WHERE username=$username);

if(mysql_num_rows($query) == '1')) {
$row = mysql_fetch_array($query);

if($row['password'] == $password) {
$_SESSION['username'] = $username;
echo "You're logged in!";
}
}
}

?>