How to create new instance of class in laravel through variable name?

Just give full path of your Model class as below:

$className = $request->model_type;
$class_path = 'App\\'.$className; // take  a full path of your Model class.
$class = new $class_path;

return $class;

You must use complete namespace:

$class = 'App\'.$className

This is correct code of snippet you've added.

And it should work with full namespace path:

if($request->has('model_type')){
  $class = '\\App\\Models\\' . (string) $request->model_type;
  
  return new $class;
}

Firstly, I believe the answers posted already are pretty good to resolve your issue.

Nonetheless, I personally feel that you're doing it the wrong way.

1. You would rarely need to pass model names in your 'API' routes.

Why? - This would essentially lead to a tight coupling between your route parameters and your model names. This forces you to change your "route signature" whenever you change your model names.

2. You typically are expected to use 'id'/'slug'/'custom column' (preferably a primary key) to allow you to identify your models. i.e:

Ex 1: Resource Routes api/events/{event}/tickets/{ticket}

Ex 2: Customizing The Key api/posts/{post:slug}

With this approach:

A. You can make use of Route Model Binding.

B. You immediately know which route parameter corresponds with which model. i.e:

// use Illuminate\Http\Request;
// use App\Models\Event;
// use App\Models\Ticket;
// Incase you use 'id' as you model's primary key.
function update(Request $request, $event, $ticket)
{
    $eventModel = Event::find($event);
    $ticketModel = Ticket::find($ticket);
}

Lastly, You can still quite easily validate route parameters since they are also part of the Request object. i.e:

$request->validate([
    "event" => "required|numeric|exists:events",
    "ticket" => "required|numeric|exists:tickets",
]);