If I change it to this:
```php
if (!isset($app['validator.validator_service_ids'])) {
    $app['validator.validator_service_ids'] = array();
}

$app['validator.validator_service_ids']['security.validator.user_password'] = 'security.validator.user_password_validator';
```
I get the error:
```
Indirect modification of overloaded element of Silex\Application has no effect
```
Which seems to be related to [this](https://bugs.php.net/bug.php?id=39449)