blanket rewrite to redirect htm, html, shtml pages to php

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We just upgraded our site from shtml pages to php, but there are links to the old shtml, and html(before that), and htm (before that).

I want to do a blanket modrewrite so that anyone looking for any page shtml, htm, or html in the site gets automatically redirected to the php equivelant of that page, and it shows as php extension in their browser window.

Thank you in advance for your help.