Can I make a universal 301 redirect rule for my site?

[absoauto]

I.e. I switch shopping carts and every page has the same path/name but now ends in .php instead of .htm.

[Felip3]

You can use Excel formulas to generate a list with all your URLs with .php....

[MickEdwards]

If you have mod_rewrite enabled on your server you could try something like this in your .htaccess file:

RewriteCond %{REQUEST_URI} ^(.).htm$
RewriteRule ^(.) /$1.php [R=301,L]