How to do this in Laravel, subquery where in

Consider this code:

Products::whereIn('id', function($query){
    $query->select('paper_type_id')
    ->from(with(new ProductCategory)->getTable())
    ->whereIn('category_id', ['223', '15'])
    ->where('active', 1);
})->get();

Have a look at the advanced where clause documentation for Fluent. Here's an example of what you're trying to achieve:

DB::table('users')
    ->whereIn('id', function($query)
    {
        $query->select(DB::raw(1))
              ->from('orders')
              ->whereRaw('orders.user_id = users.id');
    })
    ->get();

This will produce:

select * from users where id in (
    select 1 from orders where orders.user_id = users.id
)

You can use variable by using keyword "use ($category_id)"

$category_id = array('223','15');
Products::whereIn('id', function($query) use ($category_id){
   $query->select('paper_type_id')
     ->from(with(new ProductCategory)->getTable())
     ->whereIn('category_id', $category_id )
     ->where('active', 1);
})->get();