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]