A simple minimal example for preprocessing PHP template data

page.html.php

<?php

// We just preprocess some data to use for the template.
// For what we want to show, it is not important what we're doing here exactly,
// rather than the fact that this cannot be done inside a Twig template,
// which is where this PHP HTML template should be convertet to.
if ($image = filter_input(INPUT_GET, 'image') &&
		!filter_var($image, FILTER_VALIDATE_URL) === false) {

		$image_src = $image;
}

?>
<?php if (isset($image_src)) : ?>
		<div>
				<a href="http://example.com/">
						<img src="<?= addslashes($image_src) ?>"/>
				</a>
		</div>
<?php endif ?>