注册，包括后失败

我正在尝试建立一个带有注册等功能的登录系统。现在，我使用一个表格和方法"张贴"进行注册。现在它失败了，我认为是通过邮件发送输入。你能帮我修一下吗？这是其中涉及的代码：

上面！doctype

<?php
include('connect.php');
// If the values are posted, insert them into the database.
if (isset($_POST["username"]) && isset($_POST["password"])){
    $username = $_POST["username"];
    $email = $_POST["email"];
    $password = $_POST["password"];
    $query = "INSERT INTO `user` (username, password, email) VALUES ($username, $password, $email)";
    $result = mysqli_query($query);
    if($result){
        $msg = "User Created Successfully.";
    }
else
  {echo "fail";}
}
?>

形式：

<div class="register-form">
<?php
if(isset($msg) & !empty($msg)){
    echo $msg;
}
?>
<h1>Registreer</h1>
<form action="" method="POST">
<p><label>User Name : </label>
<input id="username" type="text" name="username" placeholder="username" /></p>
<p><label>E-Mail&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; : </label>
 <input id="password" type="email" name="email" required placeholder="name@email.com" /></p>
 <p><label>Password&nbsp;&nbsp; : </label>
 <input id="password" type="password" name="password" placeholder="password" /></p>
<a class="btn" href="login.php">Login</a>
<input class="btn register" type="submit" name="submit" value="Registreer" />
</form>
</div>    

connect.php

<?php
$servername = "localhost";
$username = "sqluser";
$password = "Welkom01!";
$dbname = "users";
$connection = mysqli_connect($servername, $username, $password);
if (!$connection){
die("Database Connection Failed". mysqli_error($connection));
}
$select_db = mysqli_select_db($connection, $dbname);
if (!$select_db){
die("Database Selection Failed" . mysqli_error($connection));
}
?>

提前谢谢。