登录功能总是以错误告终

由于某种原因，这段代码。。。

if(empty($_POST) === false) {
#$username = htmlentities(mysqli_real_escape_string($conn, $_POST['username']));
#$password = htmlentities(mysqli_real_escape_string($conn, $_POST['password']));
$username = $_POST['username'];
$password = $_POST['password'];
if(empty($username) === true || empty($password) === true) {
    $errors[] = 'You need to enter a username and a password';
} else if (user_exists($username) === false) {
    # code...
    $errors[] = 'We can''t find that username. Have you registered?';
} else {
    $login = login($username,$password);
    if($login === false) {
        $errors[] = 'That username/password combination is incorrect.';
    } else {
        die($login);
        $_SESSION['user_id'] = $login;
        header('Location: index.php');
        exit();
    }
}
output_errors($errors);
}

下面是函数。。不确定这里到底发生了什么，使它不真实。当然，我还在学习mysqli_*扩展，但逻辑对我来说是有意义的。这很奇怪。

function user_id_from_username($username) {
$conn = mysqli_connect("127.0.0.1","root","","lr");
$username = sanitize($username);
return mysqli_data_seek(mysqli_query($conn, "SELECT `id` FROM `users` WHERE `username` = '$username'"), 0);
mysqli_close($conn);
}
function login($username,$password) {
  $conn = mysqli_connect("127.0.0.1","root","","lr");
  $user_id = user_id_from_username($username);
  $username = sanitize($username);
  $password = md5($password);
return (mysqli_data_seek(mysqli_query($conn,"SELECT COUNT(`id`) FROM `users` WHERE `username` = '$username' AND `password` = '$password'"), 0) == 1) ? $user_id : false;
#return (mysqli_query($conn,"SELECT count('id') FROM users WHERE username = '$username' AND password = '$password'") === 1) ? $user_id : false;
}