I had website content in a subfolder (http://mydomain.com/subfolder/index.php), now I copied everything over to the root folder (http://mydomain.com/index.php).
I wo
Placing the following .htaccess in / (where your index.php is located) should do the trick:
RewriteEngine on
RewriteRule ^subfolder/(.*)$ /$1 [R=301,L]
Or you could place the following .htaccess in /subfolder:
RewriteEngine On
RewriteRule ^(.*)$ /$1 [R=301,L]
Note that the () around .* and the $1 redirects /subfolder/someFile.php to /someFile.php. If you skip it, everything in /subfolder redirects to /.