HTML forms done like dis in a PHP app for example:

<form action="form.php" method="post" id="message">
    Name: <input type="text" name="name"><br>
    E-mail: <input type="email" name="email"><br>
    <input type="submit">
</form>
 
<textarea name="message" form="message">Write message here...</textarea>

This leads to the textarea being below the submit button but still being part of the form since you can't put textareas as a form input.

Form needs to do a Post then the data can be read from the post like so:

<?= 'Name: ' . $_POST["name"] ?>
<br>
<?= 'Email: ' . $_POST["email"] ?>
<br>
<?= 'Phone: ' . $_POST["phone"] ?>
<br>
<?= 'Birthday: ' . $_POST["birthday"] ?>
<br>
<?= 'Message: ' . $_POST["message"] ?>

Added

htmlspecialchars()

around output so no special chars can be inserted.

Yoooo I made flash messages, gotta put them in the session (cuz that stays across pages) then send the user back to the index and check in the index for flash messages.

form.php

foreach ($_POST as $key => $value) {
    if (empty($value)) {
        session_start();
        $_SESSION['validation_error'] = $key . " cannot be empty";
 
        header('Location: index.php');
        exit;
    }
}

index.php

<?php
session_start();
if (isset($_SESSION['validation_error'])): ?>
    <div style="color: red">
        <?= htmlspecialchars($_SESSION['validation_error']) ?>
    </div>
    <?php unset($_SESSION['validation_error']) ?>
<?php endif ?>