Namespace conflict when using 3rd party api in class - php

Question

There is something weird going on. I'm a novice with respect to php programming.
I'm trying to use FirePHP but my question is actually not related to that tool directly.
The function fb you'll see below is part of FirePHP. My entry point is Main.php. The first fb call is executed without any problems but the second call (see ExperController.php) ,which gets triggered when $ec->exper(); in Main.php is called, causes a fatal error:
Fatal error: Uncaught Error: Class 'App\Controllers\FirePHP' not found in path-to-wp-directory\wp-content\mu-plugins\typerocket\app\Controllers\ExperController.php on line 12
Why is the runtime engine looking for the class FirePHP under that namespace (App\Controllers)? And why wasn't this an issue during the first fb call? There isn't any namespace defined in the FirePHP files at all. And my last but crucial question, how can I fix this without having to touch 3rd party files? This answer is not a solution for my case since I'm not referencing / calling the mentioned class in my code!
Note: FirePHP is included via autoloader (provided by composer).
Main.php:
<?php
fb('Hello World!', FirePHP::INFO);
$req = new \TypeRocket\Http\Request();
$res = new \TypeRocket\Http\Response();
$ec = new \App\Controllers\ExperController($req, $res);
$ec->exper();
ExperController.php:
<?php
namespace App\Controllers;
use TypeRocket\Controllers\Controller;
class ExperController extends Controller
{
public function exper() {
fb('Hello World!', FirePHP::INFO);
}
}

Answer 1

do you use any Dependency manager like composer??
when you call it from main somehow php can see where is FirePHP class is and everything works fine but when you try to access it inside a class in another folder you must define where the class is inside your controller , like how you defined where is controller class that you extend :
use TypeRocket\Controllers\Controller;
so add FirePHP class to your controller and everything will be ok then.
use Path\To\Class\FirePHP;
also if you did all and still not working you may try dump autoload;
composer dump-autoload