Creating a lightweight dismissible banner on a PHP site

lightweight-dismissible-banner.php

<?php
$cookie_name = "banner";
if(!isset($_COOKIE[$cookie_name])) :
?>

<div class="banner">
  <!-- your content -->
  <button onclick="alertClose()" aria-label="Close">
    <span role="presentation">&#10005;</span>
  </button>
</div>

<script>
function alertClose() {
    document.cookie = "<?php echo $cookie_name; ?>";
    document.getElementById("banner").style.display = "none";
}
</script>

<?php endif; ?>