Slim3 and PHPBB3

I am trying to implement a PHP forum on my website. If I wasn’t using Slim to route everything, it would be a basic drag and drop into my /public_html/ folder. But I am using Slim as a blanket router (as in, every single request to my website goes to Slim first). For example, my .htaccess in my public_html file looks like:

<IfModule mod_rewrite.c>
    RewriteEngine on
    #RewriteBase /
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteRule . index.php [QSA,L]
</IfModule>

That index.php file includes my Slim app, and the routes.php file, which handles every request very manually.

So - how do I set up Slim3 so that when a user goes to www.example.com/forum/, it lets data go to /public_html/phpbb3/, which has its own .htaccess and internal PHP routing system?

I think this is what you want.

<IfModule mod_rewrite.c>
    RewriteEngine on
    #RewriteBase /
    RewriteCond %{REQUEST_URI} !^/forum
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteRule . index.php [QSA,L]
</IfModule>

Yes, thank you very much.