In the HTML, just use the HTML character entity for the < and > characters. < is & lt; (minus the space--IPB can't display it unless there is a space) and > is & gt; (again, minus the space). So it'd look something like this:
CODE
& lt;php include('path/to/include.php'); & gt;
Or, you can use the PHP htmlentities function, which converts HTML entities:
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<?php
$example = '<?php include(\'path/to/include.php\'); ?>';
echo htmlentities($example);
?>