An application inside the website

Hello,
I have a question when I have an application inside my website, how to configure the framework.
I installed the framework via composer and copied the designated folder to my application's folder, so I can have:

localhost/ -> my website
localhost/application -> an application from codeigniter framework

So, the application folder has all those folders from the framework, and I updated the $systemDirectory variable inside Paths.php.

When I run php spark serve inside application folder, the framework works and I got the welcome message. But when I am in my localhost/ and calls application/ then the application do not run and It shows me the files of the directory. Which configurations the codeigniter framework needs to run inside another website? When I want to run application from localhost/, I have to redirect the user to which directory?

Thanks in advance.