namespace AppServices;
use Appaaaaaa;

class ApiGatewayService
{
    protected $aa;

    public function __construct()
    {

    }
    public function aa()
    {
        $this->aa = new aa();//<<< THIS LINE
        return $this->aa->aa();
    }
}

namespace Appaaaa;

class aa
{
    public function aa()
    {
        return 'original';
    }
}

public function test_aa()
{
    $aa = Mockery::mock('overload:'.aa::class);
    $aa->shouldReceive('aa')->andReturn('mocked');
    $apiGatewayService = new ApiGatewayService();
    $apiGatewayService->aa(); //return "mocked"
}