.htaccess

# Redirect Primary Domain URL to Sub-Directory using .htaccess

# Do not change this line.
RewriteEngine on

# Change domain.com to be your primary main domain.
RewriteCond %{HTTP_HOST} ^(www.)?domain.com$

# Change 'subfolder' to be the folder you want to redirect request to.
RewriteCond %{REQUEST_URI} !^/subfolder/

# Don't change this line.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

# Change 'subfolder' to be the folder you want to use for your primary domain.
RewriteRule ^(.*)$ /subfolder/$1

# Change domain.com to be your primary domain again.
# Change 'subfolder' to be the folder you will use for your primary domain
# followed by / then the main file for your site, index.php, index.html, etc.
RewriteCond %{HTTP_HOST} ^(www.)?domain.com$
RewriteRule ^(/)?$ subfolder/index.php [L]