Where to save files in Laravel 5 folder structure?

/resources are not the best place, as this folder is used for source files and is usually stored in source code repository (e.g. git).

Files that application generates usually end up somewhere in /storage folder - just create a /storage/csv folder there.

You should never reference those files directly from your classes. Laravel's filesystems are what you need - you can read more about them here: http://laravel.com/docs/master/filesystem. They make operations on the files (like read, write, prepend, append, delete, move, get all files and many more...) much simpler.

Start with defining a filesystems in your config/filesystems.php

'disks' => [
  'csv' => [
    'driver' => 'local',
    'root'   => storage_path().'/csv',
  ],
],

Now you can read/write your csv files via Storage facade from anywhere in your code like that:

Storage::disk('csv')->put('file.csv', $content);
$content = Storage::disk('csv')->get('file.csv');