For my personal taste i better like when Interface and all its concrente implementations are together in one folder, it is later easier to locate them all and see all possible implementations.

And i would name them a little bit different:

// Interface
Acme/Repositories/UserRepository

// Concrete implementations
Acme/Repositories/EloquentUserRepository
Acme/Repositories/MongoUserRepository
Acme/Repositories/SomeRemoteServiceUserRepository

But it is just me. Some say that append interfaces with word 'Interface' is betters, because you can quickly find all interfaces + just by reading file name you can know that it is an interface.

// HomeController.php

// you can only use interface, you don't need to additionally include concrete class that will be used, so you can remove fallowing:
use Acme\repositories\UserRepository as UserRepository;

// You can use 'use' like so if class name does not change
use Acme\interfaces\UserInterface;

// When to use as suffix
use Acme\interfaces\UserInterface as MyDifferentName;

// Use capitalized namespaces
use Acme\Interfaces\UserInterface;

Good start with Laravel! JMO, my suggestions would be to

• Inject the DB and User classes into the UserRepository.
• rename UserInterface to UserRepositoryInterface,
• create a new UserInterface for the User class then use that to typehint the dependency injection into the UserRepository construct.

A refactored UserRepository would look something like this:

namespace Acme\repositories;

use DB;

class UserRepository implements UserRepositoryInterface
{
	protected $db;
	protected $user;

	public function __construct(DB db, UserInterface $user)
	{
		$this->db = $db;
		$this->user = $user;
	}
	
	public function listUsers()
	{
		return $this->user->all();
	}
}