laravel load external file code example

Example 1: include a file in laravel controller

require_once __DIR__ . '/../app/Utilities/FileName.php';

Example 2: include a file in laravel controller

If you have a custom file containing some classes/functions that need to be\
  loaded for every request, you need to make sure its added to the autoloader.

In your composer.json add the following in your autoload section:

"autoload": {
  "files": [
    "path/to/your/File.php"
  ]
}

OR
  
First, make sure you have a namespace declaration at the top of your included 
file - say namespace Your\Namespace. In order to avoid conflicts, you need 
to explicitly tell PHP which class you mean when you reference it in the code. You mentioned your file contains a Response class that also exists in Laravel. In order to be able to use both, you need to alias one of them:

use Illuminate\Http\Response as LaravelResponse;
use Your\Namespace\Response;