``alias="form"`` is wrong, and it will break the form rendering (as it is registered after the real form helper, it will replace it in the engine)

@fabpot should we add a validation in the lazy PhpEngine to check that the alias is matching the real helper name (as done for form types as of Symfony 2.2) ? It would help debugging such issues. It would add 1 more check for the *first* retrieval of each helper only (as the instance itself is registered after that) so it would not have a huge overhead.