注册成功后无法登录数据库

这是我的登录码，不知道什么是错的(最后一个if，总是去最后一个else)。我什么都试过了，但还是不行。

<?php
error_reporting(E_ALL & ~E_NOTICE);
session_start();
if($_POST['submit']){
    include_once("connection.php");
    $username = strip_tags($_POST['username']);
    $password = strip_tags($_POST['password']);
    $sql = "SELECT id, username, password FROM user WHERE username = '$username' AND password = '$password'";
    $query = mysqli_query($connection, $sql);
    if($query){
        $row = mysqli_fetch_row($query);
        $userId = $row[0];
        $dbUsername = $row[1];
        $dbPassword = $row[3];
    }
    if ($username = $dbUsername && $password == $dbPassword) {
        $_SESSION['username'] = $username;
        $_SESSION['id'] = $userId;
        header('Location: users.php');
    }else {
        header('Location: error.php');
    }
}
?>

和连接代码

<?php
$connection = mysql_connect('localhost', 'root', '');
if (!$connection){
    die("Database Connection Failed" . mysql_error());
}
$select_db = mysql_select_db('login');
if (!$select_db){
    die("Database Selection Failed" . mysql_error());
}
?>