mount vars

success:
```php
$router->mount('/video', function () use ($router) {
$router->get('/([\w_\d]+)', function ($id) {
echo 'video '.$id;
});
});
```
error:
```php
$router->mount('/video/([\w_\d]+)', function ($id) use ($router) {
$router->get('/', function () use ($id) {
echo 'video '.$id;
});
});
```
why ???

Start by reproducing the two PHP examples and trace the router's mount handling, focusing on how route variables are passed into the mounted callback. Compare the successful and failing forms to determine the expected behavior and verify that the resulting explanation or fix covers both cases.

Written by the indexing model from the issue text.