Cannot declare class Controller, because the name is already in use

I'm pretty sure you have to add namespaces.

Namespacing

By default, Laravel 4 applications did not utilize namespacing within your application code. So, for example, all Eloquent models and controllers simply lived in the "global" namespace. For a quicker migration, you can simply leave these classes in the global namespace in Laravel 5 as well.

Remove the "app/Http/Controllers" from your classmap.

Add namespace App\Http\Controllers; above your use blocks.

Then run composer dump-auto