为什么这个WHERE子句不接受变量？ Laravel 5.3

I have this function here:

我在这里有这个功能:

public function move() {

        $prefs = DB::table('users')->select('id', 'Preferences')->where('Preferences', '!=', '')->get();

            foreach ($prefs as $pref) {
                 $tags = $pref->Preferences;
                 $tag = explode(',', $tags);
                 foreach ($tag as $t) {
                     $new = DB::table('prefs')->select('id', 'tag')->where('tag', '=', $t)->get();
                     $taguser = new Tag(array(
                                       'user_id' => $pref->id,
                                       'tag_id' => $t,
                                       'tagsid' => $new->id
                                      ));
                     $taguser->save(); //save in table
                 }
             }
             return view('refactor');
    }
publ