Rewrite variable to folder [closed]
(Actual url) Before: yyy.com/folder/index.php?lang=en
(Their url)  After:  yyy.com/folder/en/index.php

Before: yyy.com/folder/submit.php?lang=da
After:  yyy.com/folder/da/submit.php

RewriteEngine On
RewriteRule ^/?folder/([a-z]{2})/index.php$ /folder/index.php?lang=$1 [L,QSA]