为什么我的代码没有在mysql表中填充一行?

我试图允许用户注册，代码回来没有任何错误，但没有显示在我的数据库。我怎么也找不出原因。我没觉得有什么不对，但你们谁能帮帮我?

这是完整的php文件:

<?php
require "includes/constants.php";
$conn = new mysqli(DB_SERVER, DB_USER, DB_PASSWORD, DB_NAME);
if(isset($_POST['username'])) {
    $username = $_POST['username'];
    $uLength = strlen($username);
}
if ($uLength >= 4 && $uLength <= 15)
{
}
else
{
    die("Your username must be between 4 and 15 characters.");
}
if($username == "")
{
    die("You didn't tell me what to call you! Please enter a username.");
}
if(isset($_POST['pwd'])) {
    $pwd = $_POST['pwd'];
    $pLength = strlen($pwd);
}
if ($pLength >= 6 && $pLength <= 41)
{
}
else
{
    die("Password must be between 6 and 41 characters.");
}
if($pwd == "")
{
    die("Please enter a password and verify it.");
}
if(isset($_POST['pwd_conf'])) {
    $pwd_conf = $_POST['pwd_conf'];
}
if($pwd != $pwd_conf)
{
    die("Ouch! Your passwords don't match! Try again.");
}
if(isset($_POST['email'])) {
    $email = $_POST['email'];
    $email1 = "@";
    $email_check = strpos($email,$email1);
}
$user_check = "SELECT username FROM users WHERE username='$username'";
if($stmt = mysqli_prepare($conn,$user_check)) {
    mysqli_stmt_execute($stmt);
    if(mysqli_stmt_num_rows($stmt) > 0){
        die("Username is already in use!<br>");
    }
}
$query = "INSERT INTO users (username, password, email) VALUES ('$username', '$pwd', '$email')";
if(!$query)
{
    die("Unfortunately, we can't sign you up because we have problems: ".mysql_error());
}
else
{
    header("Location: login.php");
}
?>