How to set global variable laravel

I tried to make a global variable in laravel, in my code when the json return response, the data appears, but the other method is why it is null,

there is my code

class VendorController extends Controller
{

    private $vendor_id;

    public function index(){
    if($cek =='available')
    {
        $this->vendor_id = DB::getPdo()->lastInsertId();
        return response()->json([
            'status' => 'success',
            'vendor_id' => $this->vendor_id
        ]);
    }
    }
    public function cek(){
    dd($this->vendor_id)
    }


}

when in function cek $this->vendor_id result is null, but in index function return->response()->json() there data is 13

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