Source.php
<?php
echo
<<<HTML
<!DOCTYPE html>
<html>
<head>
<style>
html,body, body > pre {
padding:0;
margin:0;
}
body > pre > code.hljs{
margin:0;
padding:16px;
}
</style>
<link rel="stylesheet"
href="//cdn.jsdelivr.net/gh/highlightjs/cdn-release@10.1.2/build/styles/default.min.css">
<script src="//cdn.jsdelivr.net/gh/highlightjs/cdn-release@10.1.2/build/highlight.min.js"></script>
<script>hljs.initHighlightingOnLoad();</script>
</head>
<body>
<pre><code class="language-{$extension}">{$source}</code></pre>
</body>
</html>
HTML;