symfony/symfony-docs — pull request #3570 by nixilla (2014-02-12 16:28:45+00:00)
Title: Callback: [Validator, validate] expects validate to be static

Otherwise you'll get:
```
ContextErrorException: Runtime Notice: call_user_func() expects parameter 1 to be a valid callback, non-static method Vendor\Package\Validator::validate() should not be called statically in Symfony/Component/Validator/Constraints/CallbackValidator.php
```