Using htaccess change the website default index file from root folder to sub folder, site other pages should still work from root folder

Question

Using htaccess how to change the PHP website default home page ie index.php file from root folder ie "html" to sub folder(say it 'subfolder') but site should still work as before ie the home page will be from 'subfolder' but other pages will still using same old root "html" folder.

NOTE :

• Website have SSL ie (https)

• Home page Browser URL should always be https://www.mywebsite.com/

• root folder is "html".

• The directory path of "subfolder" is "html/subfolder", this folder only have only one file ie 'index.php'

• So there should be no change in any page URL (including home page).

• NO Redirect

Answer 1

As per Apache document https://httpd.apache.org/docs/2.4/mod/mod_dir.html, I tried

DirectoryIndex subfolder/index.php

So the Updated root folder .htaccess code is

<IfModule mod_rewrite.c>

Options +FollowSymLinks

RewriteEngine on
RewriteBase /app/

#--------- 
DirectoryIndex subfolder/index.php

#--------- 

RewriteRule (.*?[A-Z]+.*) ${lc:$1} [R=301,L]

RewriteCond %{HTTP_HOST} !^www\.mywebsite\.com$ [NC]
RewriteCond %{HTTPS}s ^on(s)|off    
RewriteRule ^(.*)$ http%1://www.mywebsite.com/$1 [R=301,QSA,L]    

RewriteRule    ^$    webroot/    [L]
RewriteRule    (.*) webroot/$1    [L]
RewriteRule ^(.*)&(.*)$ $1__-__$2 [N]