Load theme files from subdirectory into main index.php in php

I am trying to create an PHP application. So I want to create a theme folder and Inside theme folder , I have a theme.

Example -

root
|
|----folder1
|       
|----theme folder
|       |----theme asset
|       |        |
|       |        |----- css
|       |        |
|       |        |----- js
|       |
|       |         
|       |----header.php
|       |
|       |----footer.php
|       |
|       |----sidebar.php
|       |
|       |----index.php
|
|       
|
|----index.php

When I search http://localhost/index.php , Then it should load complete theme with header,footer and sidebar but without including theme css and js file in main index.php.

I know only core php, So someone can tell me how can i do this

Thanks in Advance