Nathan,
There certainly will be some increase in overhead, but how much depends on lots of different factors. Your best bet is probably to add .htm(l) to the PHP AddType and run some benchmarks before you start incorporating PHP code into the docs. Any easy way to handle the redirects (if needed) is to use the following redirection script:
<?php
if (stristr($HTTP_SERVER_VARS["REQUEST_URI"], ".html") || stristr($HTTP_SERVER_VARS["REQUEST_URI"], ".htm")) {
$redirect = ereg_replace(".html", ".php", $HTTP_SERVER_VARS["REQUEST_URI"]);
$redirect = ereg_replace(".html", ".php", $url);
header("Location: $redirect");
exit();
}
?>
Place this script in the auto_prepend_file directive in php.ini and PHP will automatically call this file before everypage load. If the request is for a doc with the .html or .htm extension, the user will be transparently redirected to the same file path and name, just using the .php extension. Of course, the .html or .htm file must exist (or they get a 404) and it will not distinguish between html files that need PHP and ones that don't..... but it should get you started. HTH.
geoff
