admin.php file content:
<?php
require 'index.php';
.htaccess content
<FilesMatch "^admin\.php">
php_value max_input_vars 20000
</FilesMatch>
... rest of the code ...
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond $1 ^admin/
RewriteCond $1 !^(admin\.php)
RewriteRule ^(.*)$ admin.php/$1 [L,QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond $1 !^(index\.php)
RewriteRule ^(.*)$ index.php/$1 [L,QSA]