Looking for a better way to show/hide an element

Question

I'd like to know if someone could please show me a better way to show/hide an element on my page. This is how I am currently doing it.

At the very top of my index.php file, I have the code for my application. Here is the function which determines whether the form has been submitted and populates $_SESSION['prompt'] with data.

private function validateForm() {
    if (isset($_POST['submit'])) {
        $_SESSION['prompt'] = 'Form submitted';
    }
}

And here is the element which I would like to show/hide depending on whether the form has been submitted located towards the bottom of my index.php file.

<p id="dialog">
    <?php
        if (!empty($_SESSION['prompt'])) {
            echo $_SESSION['prompt'];
        }
        else {
            echo '<script>document.getElementById("dialog").style.display="none";</script>';
        }
    ?>
</p>

Answer 1

It's hardly a great leap in creative logic to work that one out

<?php
    if (!empty($_SESSION['prompt'])) {
        echo '<p id="dialog">'.$_SESSION['prompt'].'</p>';
    }
    else {
      // nothing
    }
?>

Why are you setting $_SESSION variables that you then use in the same file, as opposed to just variables? Are you using that data later?