PHP:将值从注册表单插入MySQL表

I'm very new to PHP and have to use it for school to make a prototype login/signup page. However I am having an issue inserting values from a sign up form into a MySQL database. I have created a customers table in a UTTCv5 schema on phpmyadmin. I sourced the code from this website https://www.tutorialrepublic.com/php-tutorial/php-mysql-login-system.php and edited it to add the first name and credit card number. However, when I complete and submit the form, the data is not added to the customers table.

config.php:

<?php
/* Database credentials */
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'root');
define('DB_PASSWORD', '');
define('DB_NAME', 'utccv5');

/* Attempt to connect to MySQL database */
$link = mysqli_connect(DB_SERVER, DB_USERNAME, DB_PASSWORD, DB_NAME);

// Check connection
if($link === false){
    die("ERROR: Could not connect. " . mysqli_connect_error());
}
?>

register.php:

<?php
// Include config file
require_once "config.php";

// Define variables and initialize with empty values
$username = $password = $first_name = $credit_card = "";
$username_err = $password_err = $first_name_err = $credit_card_err = "";

// Processing form data when form is submitted
if($_SERVER["REQUEST_METHOD"] == "POST"){

    // Validate email
    if(empty(trim($_POST["username"]))){
        $username_err = "Please your email.";
    } else{
        Prepare a select statement  
        $sql = "SELECT email FROM customers WHERE email = ?";

        if($stmt = mysqli_prepare($link, $sql)){
            // Bind variables to the prepared statement as parameters
            mysqli_stmt_bind_param($stmt, "s", $param_username);

            // Set parameters
            $param_username = trim($_POST["username"]);

            // Attempt to execute the prepared statement
            if(mysqli_stmt_execute($stmt)){
                /* store result */
                mysqli_stmt_store_result($stmt);

                if(mysqli_stmt_num_rows($stmt) == 1){
                    $username_err = "This email is already taken.";
                } else{
                    $username = trim($_POST["username"]);
                }
            } else{
                echo "Oops! Something went wrong. Please try again later.";
            }

            // Close statement
            mysqli_stmt_close($stmt);
        }
    }

    // Validate password
    if(empty(trim($_POST["password"]))){
        $password_err = "Please enter a password.";     
    } elseif(strlen(trim($_POST["password"])) < 6){
        $password_err = "Password must have atleast 6 characters.";
    } else{
        $password = trim($_POST["password"]);
    }

    // Validate first name
    if(empty(trim($_POST["first_name"]))){
        $first_name_err = "Please enter your first name.";     
    } elseif(strlen(trim($_POST["first_name"])) < 1){
        $first_name_err = "Your first name must have atleast 1 character.";
    } else{
        $first_name = trim($_POST["first_name"]);
    }

    // Validate credit
    if(empty(trim($_POST["credit_card"]))){
        $credit_card_err = "Please enter your credit card number.";     
    } elseif(strlen(trim($_POST["credit_card"])) < 16){
        $credit_card_err = "Your first name must have atleast 1 character.";
    } else{
        $credit_card = trim($_POST["credit_card"]);
    }



    // Check input errors before inserting in database
    if(empty($username_err) && empty($password_err) && empty($first_name_err) && empty($credit_card_err)){

        // Prepare an insert statement
        $sql = "INSERT INTO customers (username, password, first_name, credit_card) VALUES (?, ?, ?, ?, ?, ?)";

        if($stmt = mysqli_prepare($link, $sql)){
            // Bind variables to the prepared statement as parameters
            mysqli_stmt_bind_param($stmt, "ssss", $param_username, $param_password, $param_first_name, $param_credit_card);

            // Set parameters
            $param_username = $username;
            $param_password = password_hash($password, PASSWORD_DEFAULT); // Creates a password hash
            $param_first_name = $first_name;
            $param_credit_card = $credit_card;

            // Attempt to execute the prepared statement
            if(mysqli_stmt_execute($stmt)){
                // Redirect to login page
                header("location: login.php");
            } else{
                echo "Something went wrong. Please try again later.";
            }

            // Close statement
            mysqli_stmt_close($stmt);
        }
    }

    // Close connection
    mysqli_close($link);
}
?>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Sign Up</title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.css">
    <style type="text/css">
        body{ font: 14px sans-serif; }
        .wrapper{ width: 350px; padding: 20px; }
    </style>
</head>
<body>
    <div class="wrapper">
        <h2>Sign Up</h2>
        <p>Please fill this form to create an Under the Clock account.</p>
        <form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
            <div class="form-group <?php echo (!empty($username_err)) ? 'has-error' : ''; ?>">
                <label>Email</label>
                <input type="text" name="username" class="form-control" value="<?php echo $username; ?>">
                <span class="help-block"><?php echo $username_err; ?></span>
            </div>    
            <div class="form-group <?php echo (!empty($password_err)) ? 'has-error' : ''; ?>">
                <label>Password</label>
                <input type="password" name="password" class="form-control" value="<?php echo $password; ?>">
                <span class="help-block"><?php echo $password_err; ?></span>
            </div>
            <div class="form-group <?php echo (!empty($first_name_err)) ? 'has-error' : ''; ?>">
                <label>First name</label>
                <input type="text" name="first_name" class="form-control" value="<?php echo $first_name; ?>">
                <span class="help-block"><?php echo $confirm_password_err; ?></span>
            </div>
            <div class="form-group <?php echo (!empty($credit_card_err)) ? 'has-error' : ''; ?>">
                <label>Credit Card</label>
                <input type="text" name="credit_card" class="form-control" value="<?php echo $credit_card; ?>">
                <span class="help-block"><?php echo $credit_card_err; ?></span>
            </div>
            <div class="form-group">
                <input type="submit" class="btn btn-primary" value="Submit">
                <input type="reset" class="btn btn-default" value="Reset">
            </div>
            <p>Already have an account? <a href="login.php">Login here</a>.</p>
        </form>
    </div>    
</body>
</html>
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