如何在我的目录结构中从javascript访问我的php文件？

I have the following directory structure in my PHP project (framework: https://github.com/panique/mini)

application
--controller
----album.php
--libs
----s3demo.php
--model

public
--css
--img
--js
----application.js

In the application.js (inside js folder) I have the following code

    signature: {
        endpoint: "s3demo.php"
    },

How can I access the s3demo.php (in the libs folder) from my javascript (application.js in the js folder)? I have tested with relative path ../../application/libs/s3demo.php and even tried to go through my controller album.php but cant access the file.

s3demo.php includes access codes to my Amazon S3 account so I want to have it outside my public folder.

Any suggestion?

Edit

After I read the documentation (again) I changed the javascript to

signature: {
    endpoint: url + "/album/uploadImage"
},

And created a new function in album.php

public function uploadImage() {
    return APP . '/libs/s3demo.php';
}

But this does not work either. I think I understand routes. The javascript calls the function in album.php and the funtion return the s3demo.php. But it feels wrong to return a php-file.