Hey @zerodps fyi you are not using Eloquent but the QueryBuilder atm. So you can change what you need to bind with a question mark ? and then pass the value as second parameter:
DB::select('select projects.*, users.*, thumbnails.* from thumbnails inner join projects, users where projects.id = thumbnails.id && users.id_user = ?', [$this_user]);
To do it more of an Eloquent way you should have models created and the relationships setup within, more on this in the documentation.
thank you very much for your insight, as i mentioned the ORM topic is not my strongest topic :)
I really need to take a closer look at the documentation !