Detect if attach or detach Laravel

I have project and user table, i want to update attached user in project so i want to attach more or remove user to project

My code look like this

 $project = Project::find($id);
            $project->update($request->all());
            if ($project->users()->where('users.id',$request->input('user_id'))->count() == 0){
                $project->users()->attach($request->input('user_id'));
            }

            else if($project->users()->where('users.id',$request->input('user_id'))->count() != 0){
                $project->users()->detach($request->input('user_id'));
            }
            return response()->json($project);

This code not work correctly when add else if for detach user that want, so anyone help me to resolve this issue and thanks

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