"Undefined variable: data"

I was making a market plugin. I created my "cases" under the switch($data) value in my form.
I got this error when I tried the plugin. What should I do to fix the error?

HTML:

[22:58:03] [Server thread/CRITICAL]: ErrorException: "Undefined variable: data"
(EXCEPTION) in "plugins/[ MarketUI ]/src/MarketUI/AnaForm" at line 33

PHP:

if($data == null){
               $sender->sendMessage("§7«============[ §aMarket §7]============»\n§cMarket Menusunden Basariyla Ciktin...\n§7«=================================»");
            return;
           };

            switch($data){
                case 0:
                    $form = Dogal;
                    $form->DogalForm($sender);
                break;
                   
                case 1:
                    $form = Insaat;
                    $form->InsaatForm($sender);
                break;
                   
                case 2:
                    $form = Ekipman;
                    $form->EkipmanForm($sender);
                break;
                   
                case 3:
                    $form = Zirhlar;
                    $form->ZirhlarForm($sender);
                break;
                   
                case 4:
                    $form = Tohumlar;
                    $form->TohumlarForm($sender);
                break;
                   
                case 5:
                    $form = Diger;
                    $form->DigerForm($sender);
                break;
                   
                case 6:
                    $sender->sendMessage("§7«============[ §aMarket §7]============»\n§cMarket Menusunden Basariyla Ciktin...\n§7«=================================»");
                    return;
                break;

            };


 

Define the variable, because it is undefined

 

Now this code gives me a question...
Since I'm not very familiar with PHP, what does php detect "Dogal" in the following code as?

PHP:

$form = Dogal;


Is it detected as a string? Or...does it give you an error :smirk:

Edit: Okay so apparently it does return a string when dumped.

WHAT???

 

WHATTT