I'm pretty curious how someone could leave a backdoor in a Laravel application? In my opinion there isn't a way, because the entry point of the requests is a route file, and you have to write code for the route in order for there to be something to execute. Your team member will be able to track it down, and will tell you "Hey! What's that?".
Every request has an entry point and that entry point is a route in this case, Which will call a specific controller and its method. And there you have some logic that can be executable.
As we know that you have to follow some pretty clean rules for writing SQL queries, CSRF protection is already there. The common security vulnerabilities are well treated in the Laravel framework.
So what are the possible ways to leave a backdoor in a web application?