$_SESSION messages not displaying - php

Question

Please, I dont know what might be the problem that my $_SESSION['error'] does not display in my html file. I dont see what might be the problem.
I have error messages in session but none of them displays
in my html, i included my backend code that is php and session is started there
//HTML FILE
register.php
<? php
require_once 'php.php';
?>
<h4>Registration</h4>
<div><?php
if ( isset($_SESSION['error']) ) {
echo '<p style="color:red">'.$_SESSION['error']."</p>\n";
unset($_SESSION['error']);
}
if ( isset($_SESSION['success']) ) {
echo '<p style="color:green">'.$_SESSION['success']."</p>\n";
unset($_SESSION['success']);
}
?>
</div>
<form action="about.php" method="post" enctype="multipart/form-data">
<p id="head"></p>
<!-- <div class="mb-3"> -->
<label for="surname" class="form-label">Surname</label>
<input class="form-control" type="text" id="surname" name="surname"
<label for="firstname" class="form-label">First name</label>
<input class="form-control" type="text" id="firstname" name="firstname" value="<?= htmlentities($firstname) ?>" required>
<label for="othername" class="form-label">Othername</label>
<input class="form-control" type="text" id="othername" name="othername" value="<?= htmlentities($othername) ?>"><br>
<button type="submit" id="submitform" name="signup-btn" class="btn btn-primary btn-inline-block btn-lg">Sign Up</button>
</form>
//PHP
<?php
session_start();
require_once "pdo.php";
if (isset($_POST['signup-btn'])) {
if (strlen($_POST['surname']) < 1) {
$_SESSION['error'] = "Please, add surname";
header("Location:register.php");
return;
}
if (strlen($_POST['firstname']) < 1) {
$_SESSION['error'] = "Please, add your first name";
header("Location:register.php");
return;
}

Answer 1

The session_start must be before any HTML is emitted. In the file, you have <h4> before session_start is executed.
Shift the session_start to the top of the file. If php.php has no HTML then it can be after the include, otherwise, put it before the include.