PHP sends plain HTML to the browser by default. Web browsers don't understand PHP at all, they understand the HTML output generated by the server running the PHP script.
That said, either closing the PHP tag like this:
<?
// Some PHP.
?>
<!-- Some HTML -->
<a href='non-framed.php' target='_top'>Non-framed version</a>
<?
// Some PHP.
?>
Or just simply echoing it:
echo "<a href='non-framed.php' target='_top'>Non-framed version</a>\n";