PHP注册表没有保存

我有一个HTML注册表单，允许新用户注册到网站。：

<form action="register.php" method="POST" class="register-form">
    <h1>Create Account</h1>
    <label>
        <span>First Name :</span>
        <input id="firstname" type="text" name="firstname" placeholder="Your First Name" autocomplete="off" required/>
    </label>
    <label>
        <span>Surname :</span>
        <input id="surname" type="text" name="surname" placeholder="Your Surname" autocomplete="off" required/>
    </label>
    <label>
        <span>Username :</span>
        <input id="username" type="text" name="username" placeholder="Your Chosen Username" autocomplete="off" required/>
    </label>
    <label>
        <span>Email :</span>
        <input id="email" type="email" name="email" placeholder="Your Email Address" autocomplete="off" required/>
    </label>
    <label>
        <span>Password :</span>
        <input id="password" type="password" name="password" placeholder="Your Chosen Password" autocomplete="off" required/>
    </label>
    <hr>
    <input name="action" type="hidden" value="signup" />
    <input type="submit" class="btn register btn-success btn-lg" name="submit" value="Register">
</form>

进入register.php：

<?php
$connection = mysql_connect('localhost', 'root', 'password');
if (!$connection) {
    die("Database Connection Failed" . mysql_error());
}
$select_db = mysql_select_db('gmaps1');
if (!$select_db) {
    die("Database Selection Failed" . mysql_error());
}
// If the values are posted, insert them into the database.
if (isset($_POST['username']) && isset($_POST['password'])) {
    $firstname = $_POST['firstname'];
    $surname = $_POST['surname'];
    $username = $_POST['username'];
    $email = $_POST['email'];
    $password = $_POST['password'];

    $query = "INSERT INTO `users` (firstname, surname, username, password, email) VALUES ('$firstname', '$surname', '$username', '$password', '$email')";
    $result = mysql_query($query);
    if ($result) {
        header("Location: thank_you.html");
    } else {
        echo 'User Not Created';
    }
}
?>

但当我点击"注册"按钮时，它不会保存数据，并返回"用户未创建"。使用MySQLi比MySQL更好吗？或者有更好的方法吗？？