用Php-Mysqli更新信息

i'm watching a tutorials about CMS with OOP - PHP & RainTPL

when i try something like http://localhost/cms/admin/updateuser.php?id=4

it has ID so if(isset($_GET['id']) && (int)$_GET['id']>0) will be executed

but ( else ) executed too because it update the user !!

    public function UpdateUser()
    {
       if(isset($_GET['id']) && (int)$_GET['id']>0)
       {
           $id = (int)$_GET['id'];
           $user = $this->usersModel->GetById($id);
           System::Get('tpl')->assign($user);
           System::Get('tpl')->draw('updateuser');
       }
       else
       {
           if(isset($_POST['UpdateUser']))
           {
                //Variables
               $name       = $_POST['name'];
               $username   = $_POST['username'];
               $email      = $_POST['email'];
               $admin      = (int)$_POST['admin'];
               $id         = (int) $_POST['id'];

               //Validation


               //array Data

               $data = array(
                   'name'      => $name,
                   'username'  => $username,
                   'email'     => $email,
                   'is_admin'  => $admin
               );

               if($this->usersModel->Update($id,$data))
               {
                   System::Get('tpl')->assign('message','User Updated');
                   System::Get('tpl')->draw('success');
               }
               else
               {
                   System::Get('tpl')->assign('message','Error Updating User');
                   System::Get('tpl')->draw('error');
               }


           }
           else
           {
               //No Id
               System::Get('tpl')->assign('message','NO USER CHOSEN');
               System::Get('tpl')->draw('error');
           }

       }
   }

if(isset($_GET['id']) && (int)$_GET['id']>0)

will be executed & else too how in same time !!!!

when i try something like this http://localhost/cms/admin/updateuser.php?id=4

if statement should be executed without else statement because it can Get the id from the url

if(isset($_GET['id']) && (int)$_GET['id']>0) 

i need someone explain how does it work ?