HCI-Task-2-Sign Up

<?php require_once('connection.php'); ?>
<?php
function error($error) {
    header('Location: sign_up.php?error=' . $error);
    exit();
}


if (isset($_POST['submit'])) {
    $username = $_POST['username'];
    $firstName = $_POST['firstName'];
    $secondName = $_POST['secondName'];
    $email = $_POST['email'];

    if ($_POST['password1'] == $_POST['password2']) {
       $password = password_hash($_POST['password1'], PASSWORD_DEFAULT);
       if ( ctype_alnum($username)) {
        if ( ctype_alpha($firstName) OR ctype_alpha($secondName)) {
            if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
                $sql = "INSERT INTO users (username, first_name, second_name, user_email, user_password) VALUES ('$username', '$firstName', '$secondName', '$email', '$password')";
                if ($conn->query($sql) === TRUE) {
                    echo "Sign Up Successful";
                } else {
                    error('input');
                    // echo "Error: " . $sql . "<br>" . $conn->error;
                }
                $conn->close();

            }
            else {
                error('email');
            }
        }
        else {
            error('name');
        }
       }
       else {
        error('username');
       }
    }
    else {
        error('password');
    }
}
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Sign Up</title>
    <link rel="stylesheet" href="../styles/main.css">
</head>
<body>
    <div class="main-container">
        <form action="" method="post">
            <input type="text" name="username" placeholder="Username" maxlength="18" required>
            <input type="text" name="firstName" placeholder="First Name" maxlength="18" required>
            <input type="text" name="secondName" placeholder="Second Name" maxlength="18" required>
            <input type="email" name="email" placeholder="Email" required>
            <input type="password" name="password1" placeholder="Password" required>
            <input type="password" name="password2" placeholder="Re-type Password" required>
            <br>
            <input type="submit" name="submit" value="Submit">
        </form>
    </div>
</body>
</html>