I don't recommend using mod_rewrite because this will create duplicate content, which google doesn't like. For example, the .php page and the .phtml page will both bring up the exact same content, and this could cause problems. Instead, I would use .htaccess to create 301 Permanent Redirects. These are used when content has moved locations (to a new filename, for example). It tells the search engines that upon reaching the old URL to redirect without penalty to the new URL.
cscgal
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